"That you, by his poverty, might become rich"
The clue is in the words, "that you, by his poverty, might become rich." By what kind of poverty of Jesus were Christians made rich? By His physical poorness were we made millioniares? No, rather, by His poorness of spirit — His humility — (Matthew 5:3) we were made rich in spirit:
Revelation 3:18-20 I counsel thee to buy of me gold tried by the fire, that thou mayest be made rich; and mayest be clothed in white garments, and that the shame of thy nakedness may not appear; and anoint thy eyes with eyesalve, that thou mayest see. 19 Such as I love, I rebuke and chastise. Be zealous therefore, and repent. 20 Behold, I stand at the gate, and knock. If any man shall hear my voice, and open to me the door, I will come in to him, and will sup with him, and he with me.
This is obviously not about the richess of physical gold, or physical sight, or physical nakedness, but spiritual gold:
Sirach 33:34 I have opened my mouth, and have spoken: buy [Wisdom] for yourselves without silver, 34 And submit your neck to the yoke, and let your soul receive discipline: for she is near at hand to be found.
Which are appropriated and available through His sacrifice, which enables mercy, and thus enables repentance (which is an avenue to His grace).
The Richness of Grace vs. the Poverty of the Lack Thereof
Revelation 2:8-9 And to the angel of the church of Smyrna write: These things saith the First and the Last, who was dead, and is alive: 9 I know thy tribulation and thy poverty, but thou art rich: and thou art blasphemed by them that say they are Jews and are not, but are the synagogue of Satan.
Revelation 3:14-17 And to the angel of the church of Laodicea, write: These things saith the Amen, the faithful and true witness, who is the beginning of the creation of God: 15 I know thy works, that thou art neither cold, nor hot. I would thou wert cold, or hot. 16 But because thou art lukewarm, and neither cold, nor hot, I will begin to vomit thee out of my mouth. 17 Because thou sayest: I am rich, and made wealthy, and have need of nothing: and knowest not, that thou art wretched, and miserable, and poor, and blind, and naked.
Christ was rich because He was and is God, and didn't need to subject Himself to earthly poverty, or even just the povery (in contrast to Godhood) of becoming man, but did so out of love, so that we, who are not God, might recieve something of the blessings natural to Him by nature, through grace:
2 Corinthians 5:21 Him, who knew no sin, he hath made sin for us, that we might be made the righteous of God in him.
Philippians 2:5-11 For let this mind be in you, which was also in Christ Jesus: 6 Who being in the form of God, thought it not robbery to be equal with God: 7 But emptied himself, taking the form of a servant, being made in the likeness of men, and in habit found as a man. 8 He humbled himself, becoming obedient unto death, even to the death of the cross. 9 For which cause God also hath exalted him, and hath given him a name which is above all names: 10 That in the name of Jesus every knee should bow, of those that are in heaven, on earth, and under the earth: 11 And that every tongue should confess that Jesus is Lord, to the glory of God the Father.
Even though He was in the form of God (rich) he became a servant (poor). The sense is obvious. In doing so, and offering His life a sacrifice for sin, he opens the door of grace to us, which is a treasure, and riches.
The kingdom which God has promised to the elect is the true riches rewarded to those "rich in faith," not rich in this world.
James 2:5 Hearken, my dearest brethren: hath not God chosen the poor in this world, rich in faith, and heirs of the kingdom which God hath promised to them that love him?
Evidence for Jesus' Earthly Poverty
The Offering of the Poor
We know Jesus' parents were not rich, because they could only afford either a pair of turtle doves or two pigeons for the sacrificial offering for the dedication of the firstborn.
Luke 2:21-24 And after eight days were accomplished, that the child should be circumcised, his name was called Jesus, which was called by the angel, before he was conceived in the womb. 22 And after the days of her purification, according to the law of Moses, were accomplished, they carried him to Jerusalem, to present him to the Lord: 23 As it is written in the law of the Lord: Every male opening the womb shall be called holy to the Lord: 24 And to offer a sacrifice, according as it is written in the law of the Lord, a pair of turtledoves, or two young pigeons:
This was the offering of the "poor," according to Leviticus 14.
Isn't He Just a Carpenter?
And Jesus could hardly have became 'rich' in his simple and presumably common trade (that of the tekton — a carpenter or builder). Nor would it suit any purpose for Him or in His ministry to be such.
Matthew 13:54-56 And coming into his own country, he taught them in their synagogues, so that they wondered and said: How came this man by this wisdom and miracles? 55 Is not this the carpenter's son? Is not his mother called Mary, and his brethren James, and Joseph, and Simon, and Jude: 56 And his sisters, are they not all with us? Whence therefore hath he all these things?
If Joseph was poor and could not pay for the full sacrifice for the dedication of Jesus, then can we imagine that Jesus who worked in the same trade was any better off?
Animals Have Their Homes, but the Christ Does Not Have a Bed To Sleep In
Matthew 8:19-20 And a certain scribe came and said to him: Master, I will follow thee whithersoever thou shalt go. 20 And Jesus saith to him: The foxes have holes, and the birds of the air nests: but the son of man hath not where to lay his head.
Whereas this could have the meaning that Jesus was not welcomed into anyone's home, it could also mean that he had no place to stay — nor the money to afford the 'rent' of any place to stay.
Jesus' Expensive Tunic
Some people might cite the fact that He wore a "seamless tunic, woven top to bottom" (John 19:23) seemingly of so much value or worth that the Roman soldiers wanted it, and cast lots for it (i.e. proving He was rich), as strong proof that He was 'rich.' However, this could be the masterwork of Mary, who according to a tradition, was one of the virgins who wove for the Temple (namely, the curtain) before committed to the care of Joseph (who, because of the kind of care necessary, must not merely be a guardian, but married, to avoid ill suspicion).
Whereas some people who are generally skeptical of anything not spelled out in Scripture, this is absolutely in concordance with clues in the New Testament. First, Mary is seen to for some reason believe intercourse to be out of the question when Gabriel announces she will (future tense) conceive a son who will be the Messiah: "How shall this be done, since I know not man?" ("to know" is a Hebrew euphemism for "have sex"). Second, in Mary's Song, she, like Hannah, praises God for "Looking upon the humiliation of his handmaid." This was virtually a technical phrase in this context meaning 'God has miraculously taken away my barrenness' — in a context like Mary's this would have to be either some vow or a necessity to remain a virgin which followed her sacred status in the Temple, because barrenness is out of the question (she hadn't "known" Joseph in order to determine barrenness).
Why would she leave the Temple at all? Well, at around 12-13, most girls begin menstruating, and this makes one ritually (not morally) unclean — unfit for presence at the Temple. It might also explain Joseph's apparent old age (i.e. especially in comparison to the young virgin), since he seems to die before Jesus' crucifixion, inasmuch he is nowhere spoken of or mentioned, and Mary is entrusted to the care of one of Jesus' disciples, and taken to his house, which is unthinkable if Joseph was still alive. This despite Mary being present at the cross, and at Pentecost.
We also read in the pre-Christian Maccabean literature that in the Temple there were "hai katakleistoi ton parthenon" or "the virgins that were shut up." We even speak of "cloistered nuns" today. The Talmud also mentions the veil being woven by "eighty-two young girls" (although there is a variant in the passage). We also read in the Old Testament that women "served" or "watched" at the door of the Tabernacle, which is distinctly liturgical language used of Levitical priests elsewhere.
All this to say that there was a category that allows us to explain a very nicely — even expertly — woven garment made for Jesus by His mother, without assuming she or He was rich. In fact, similar language is used of the veil and of Jesus' garment, inasmuch as the veil of the earthly Temple was rent "top to bottom" (apo anothen eos kato) and the garment was made seamless top to bottom (ek ton anothen di holou). This explanation is overlooked because it's made a controversial issue when it should not be seen as one. Besides, isn't it also fitting that she would weave the veil for Jesus' body, the new Temple?
Virtually Everything Jesus Uses Was Borrowed Or Donated
The Upper Room (Matthew 26:17-19), His Tomb (Luke 23:50-56), a donkey (Matthew 21:1-5), etc. This is not the way of someone who is rich.