PCGS "50 States Quarters" Pricing Factors

I am relatively new to the hobby and to CCF—a few months involvement in the former and of lurking hereabouts—and am particularly attracted to proof coins. As a newbie, I assumed that TPG-slabbed coins would lend themselves to sight-unseen online purchases, would hold their value, and might be somewhat liquid. Among the coins I have acquired are the 50 States Quarters, where I am two coins away from having a clad PR69DC set. I was surprised to learn that clad coins are not infrequently more highly valued than their 90% silver counterparts. Any insights into why that might be?

Continuing: I use the PCGS set registry service to catalog these coins, which alerts the user to changes in their valuations. Many of the clad quarters went from $17.00 to $16.00 within the last week, which is not a big deal, but other reductions have been more significant. One example would be the 2005-S 25C Kansas Silver, DC (PCGS number 913064), which in PR70DC went from a putative value of $260 to $90.

The PCGS population report for this coin showed a total of 6,980 on 01 April 2015, of which 6,407 were PR69DC and 510 were PR70DC. Yesterday, the total count had increased by 100, of which 80 were PR69DC and the remaining 20 were the higher grade. This two month population increase of 1.4% wouldn't seem to warrant an almost two-thirds decrease in value. What else is going on here? Are many PCGS coins ridiculously overvalued to begin with?

Thanks for your thoughts.