I watched a lot of eBay auctions earlier this summer, and this is the general sense I get...here's approximately what you'll buy them for:
D-25 - $50-$100+ (variable depending on whether it's eBay or not)
D-35 - $75
D-211 - $75
D-303 - $175 (can be found more expensive or much cheaper)
D-311 - $225
D-555 - $150
D-777 - $175
D-E905 - $225
Denon DCP-100 - $100-$150
Denon DCP-150 - never seen sold
The D-25 isn't worth any thing more than $40 to me. The D-303 is probably worth more than it sometimes goes for because it has the best line out this side of the high Denon DCPs. The 100 and 150 both are supposedly great through the line-out. The 777 and E905 are prized for their combination of portability and excellent sound, and are probably worth what they sell for. The D-311 probably has the best combination of headphone out and line out sound and is worth it for that reason.
The D-303 may be the best value, because sometimes is goes for real cheap. Getting it for anything under $100 is a steal (and it's gone for that cheap). It's not portable though. Go for the D-777 for portability since it's a bit more available and less coveted than the E905.
That's my...11 cents or so.