Yikes! What is happening with proof state quarter sets? Several sets are now under $4. They have been weak and declining for months now, since the wind down of that program by the Mint.
However there is an underlying factor contributing to an oversupply. Many modern sets are being broken up because the silver has increased in value or there is demand for Kennedy halves and Lincolns, when this happens there is an increase in supply of the other items.
It takes new collectors and/or marketing companies to dry up such a supply, but most already have the quarters.
Be careful on any sets containing silver coins as the charts are behind in this issue because silver is moving up rapidly and buy/sell data is a day or two behind. In general, most non-silver proof sets are steady, but several mint sets of the last two decades are lower.
Morgan and Peace dollars are strong with some more aggressive bidding on the part of large trading houses. In the better date category the early scarcer Carson City issues are avidly sought in MS-60 to MS-63. The rarest issue (1893-S) has gained in Mint State as well. There is a deep pocketed individual bidding a cool million for a top graded MS-67 example. You could corner the 50 state proof quarter market with that amount of dough and your downside would be face value!