You mentioned both PTS and BTS, so I'm a bit confused, but if you're talking prehensile tailed skinks (Corucia zebrata), I have some guesses as to what's going on.
First - importation of C. zebrata is not illegal, per se. They are a CITES appendix II animal, meaning that only a limited number of individuals can be exported from the Solomon's each year, and those must have proper CITES paperwork completed at the export point and the import point.
Second - there has been a recent shipment of (supposedly) CITES documented C. zebrata into the US.
Here's where my guessing comes in: With a flood of C. zebrata coming in, dealers are looking to part with them quickly and so have lowered the price, to get them to move. Increased supply lowers prices. Plus there are at least 4-5 dealers that I've seen on the KS classifieds alone trying to move PTS. I imagine there are others out there too that simply don't advertise here. Competition may be another factor lowering prices.
If you are considering buying a WC PTS and know all of the possible negative consequences and are willing to accept them, I suggest you contact USFW DMA to check on the authenticity of the CITES paperwork. This advice comes from an individual on the Herp Law/CITES forum who seems like they'd be willing to give more info if you need it.
I'd be very interested to hear your results should you pursue purchasing one of these animals.
-Z