I think its because less and less of them exist. 10 years ago I was looking for a PowerBook 520c and there were about 4 pages of them. Probably not a single one listed on eBay now. I think in the early 2000s a lot of old Macs ended up in e-waste recycling centers or dumpsters since they didn't hold any computing value, and most Macs from the Scully, Spindler, & Amelio era wern't exactly looked at as collectors items. Now, its changed quite a bit and whats left of those old working Macs has become worth money again. I'm personally kinda glad I've kept ahold of so many 90's and 80's vintage Macs. A few years ago, I couldn't give them away, but now eBay has them listed and people are actually bidding.
I didn't notice a 520c in your signature. Here's one for a paltry $100, shipping included: http://www.ebay.com/itm/Apple-PowerBook-520c-with-PowerPC-Upgrade-RARE-/230837089752?pt=Apple_Laptops&hash=item35bef675d8#ht_500wt_1208
Just in case you were still interested