The Droid Pro is a bit harder to find on the used market than the X or the Fascinate. It's either that they really didn't sell all that many or that people who bought them are keeping them. I think it's probably a little of both. Scarcity definitely keeps the prices up a bit. It's also newer than the X or the Fascinate. Unless a phone is total crap, age matters. A newer phone is more likely to be in a better shape all around and have a longer life to someone buying it used. The global capabilities also make it more attractive to many, increasing the value (especially if it's been SIM unlocked). Neither the X or the Fascinate have that. The import market for a Droid Pro that's been SIM unlocked can fetch rather high prices (although that's starting to change with the release of the Motorola Pro in some places in Europe).
$100 - $150 for one in good shape is definitely going to be tough to come by unless someone is pretty desperate. $200 - $250 is much more likely. Good luck with your search. It's a fantastic phone that was made all the better by the recent system update.