I agree that it is unsurprising that sparrow prices could vary. What is more intriguing to me though is the tie back to the synoptic problem in the OP.
Why is it that Matthew & Luke (which almost all students of the synoptic problem acknowledge have some form of literary relationship) get their information either one from the other or both from a common source, and this seemingly trivial detail is different? Let's look at a few synoptic hypotheses and how they may or may not make sense of this.
I don't think Q does anything to simplify this problem. With or without Q, the problem remains. Somebody changed the price of the sparrow whether they got it from another Gospel or from Q. And it won't do to say there were 2 different versions of Q floating around, because then all we've done is shift the same problem back one step in the chain--we still have to explain the change, whether it was made by Matthew, Luke, or Q.
Another difficulty with appealing to Q is that, as with many synoptic questions, because it is hypothetical we can make it say anything we want. Since we do not know what it said (if it existed), hypothesizing a particular reading in Q just means we've added one more unknown to the equation, thereby increasing the number of places in our analysis where we can get it wrong. Now it's not just why did Luke change Matthew's account, but did Luke change Q, or did Matthew change Q, or did Q 2.0 change Q 1.0, or all of the above, etc.
The hypotheticals multiply quickly. Appealing to Q requires us to answer more questions than do other hypotheses. I believe that the more occasions we give ourselves to guess, the more occasions we will get the answer wrong.
Farrer or 2-Gospel Hypotheses
Both of these hypotheses reject Q and claim that Luke used Matthew as a source. In this case, Luke repeats Matthew's account and keeps a number of details, but changes the price.
At least on this assumption we have a pretty good idea where the change was made. But why would Luke have changed the price? I suggest that without additional information we can only speculate. Luke's attention to detail (how deep was the water again?) suggests it's at least possible that he checked the prevailing price and sparrows and "corrected" Matthew's account. But if he believed Jesus said it, why correct it just because the price changed?
Surely he must have known not all merchants charged the same price for sparrows all the time.
A possible clue from the Hebrew language
This is by no means a certain explanation. But it appears to me to be the simplest one.
We may have a clue available in the Hebrew of Shem Tob Matthew. If Shem Tob Matthew is not a translation from Greek, but rather a descendant of an original Hebrew Matthew (as many of its prominent students have argued) that has been corrupted but never translated, then Shem Tob Matthew 10:29 is very interesting with respect to this question.
Both Greek Matthew & Greek Luke reference the coin known as an "assarion" (sometimes called an "as"). This was an actual monetary denomination that, depending on the time period, represented some fraction of a denarius. There's a nice summary of the coinage on Wikipedia, which also points out that the assarion "was the lowest valued coin regularly issued during the Roman Empire".
The Hebrew of Shem Tob does not reference a specific denomination of coin. The word is בפרוטה, which George Howard carefully rendered as "for a small coin." (see here p. 31).
Now the Hebrew Matthew that was handed down to Shem Tob might have just dropped the reference to the Roman coinage over the years...but it's worth pointing out that this theory runs contrary to the balance of the evidence: the Greek Gospels continued to reference a Roman coin long after the empire was gone.
A simple reconstruction of what was said
This opens up what to me is a very intriguing and decidedly simple conclusion: the original statement by Jesus did not specify a denomination of coin (and therefore didn't quote a market price), but merely pointed out that at a minimal cost a pair of sparrows could be purchased. Shem Tob then would be preserving the statement as originally rendered.
Luke comes along, writing to educated Greek-speakers, and adds the actual name of the lowest-denomination coin. To his audience the 1st century version of "for a penny" would probably come off better than "for a small coin". But now he has made a generic statement into an actual price and, if he knows what sparrows sell for, he includes the actual cost.
The message went from an original statement that was generic (they cost less than a small coin) to a subsequent rendition that was specific (they cost 0.4 assarions).
The effect of translation
If Matthew was originally written in Hebrew then what we have in Greek is a translation. When Matthew was translated into Greek:
- The translator faithfully preserved the two-for-one statement in the document he was translating (ie the translator is being accurate).
- The translator was aware that either in oral retelling (in Greek), or already documented in writing by Luke, or both, the word assarion was used in place of בפרוטה when recounting this story.
- The translator did not concern himself with the latest market value of sparrows, he was just translating what his source said and used the word assarion because it was a familiar and appropriate way to render in Greek the idea of a small or low-value coin.
The solution I have offered is not a deductive proof; it's an effort to offer the simplest possible explanation--not multiplying entities beyond necessity--based on the words used in the surviving Hebrew & Greek texts.
I suggest that Jesus taught this message without quoting prices or names of Roman coins (his message was about value not price). In presenting the passage in Greek Luke added both details for purposes of precision, and the translator of Matthew stayed faithful to the sense of the original, while using the actual Greek word that had been previously used by Luke.
The kicker for me is this: these passages in Matthew & Luke are the only places in the entire Bible where forms of ἀσσάριον (assarion) are used.
This suggests that one of the Evangelists got their fractions from somewhere else (like maybe Hebrew Matthew), but one Greek Gospel directly borrowed the word assarion from the other. For one to have followed the other so closely in one detail (the coinage) suggests that the divergence in the related detail (the fractions) sitting right next to it on the scroll, requires something like Hebrew Matthew to allow for a clean explanation.
I should acknowledge that for me this hypothesis presents no significant challenge because I already believe, on other grounds, that the Gospel of Matthew was originally written in Hebrew. A summary of my thoughts on how Hebrew Matthew relates to the Greek Gospels can be found here, and a very detailed discussion is here.
I also acknowledge there are other viable hypotheses. I've simply tried to reconstruct a viable hypothesis using only known textual variants.