Shop Goodwill currently has a 600 and 900 (both with OEM lighted cases) up for auction. Both appear to be in good condition (especially the 900) and I'm guessing they'll go for under $20 with s/h. The red 600 has come up for auction at least three times now without a bidder. If these had good screens they would be great -- as is (in my opinion) they're only good for completing a Sony collection. A PRS-700 rolled off without a bid and it will probably show up again, as well.
https://www.shopgoodwill.com/Listing...uyerPrefs=true