The word θησαυροὺς, translated as "treasures," is both plural and means "wealth." If that is not sufficient attestation to the gifts' value, other evidences, both cultural and circumstantial, can help to affirm this.
The Gifts Themselves Were Precious
Gold ranks among the most costly of precious things. It is often used as coin, for payment of debt and for accumulation of wealth. Myrrh is mentioned in only one other place in the New Testament--the occasion of the burial of Jesus (see John 19:39-40), when the wealthy Nicodemus brought it for Jesus' body. Frankincense, along with gold, is mentioned in a list of precious things in Revelation 18:12-13.
The Gifts Supported the Sojourn in Egypt
For quite awhile, perhaps several years, Joseph, Mary, and Jesus were forced to stay in Egypt until Herod had died and they were no longer threatened by him. During this time he would have had little opportunity for gainful employment. As a carpenter, Joseph's tools of the trade were all in his shop back home.
The gifts were from noble guests and given to a royal recipient. The givers had come both to honor and to worship the One to whom they gave their gifts, and being wealthy themselves, they would most certainly have given of their best. While the wisemen were philosophers, not kings as many often suppose, they were of noble birth and of the wealthy class. (Think of Daniel in Babylon--also a wiseman.) Their gifts would have been representative of their status.