I've seen Zak Mc Kracken go for 280 US$ on Ebay. Also, Apidya is about 70 - 80 Euros nowadays. I got a hold on "The Patrician" and learned its usually about 50 US$. Amberstar, Ambermoon and Eye of the Beholder I/II go for insane prices as well sometimes.
Dune II ... still trying to get a hold on that one :/
- G
Edit:
I just bought Indy IV (Fate of Atlantis) in mint condition for 15$, so good luck to the guy who wants several hundred for it...
Edit2:
I had an offer for 45$ for Maniac Mansion in mint condition, but couldn't get over myself to pay that much at the moment...