it's been so long since I've kept up with them. Pretty sure the Rappan Athuk, Abysthor single modules fell far in price. I don't think the M series/Maze of Zayene ones go for much either. Maybe use NK's pricing, divide by 3 and see if it's worth it?
Noble Knight Games Role Playing Games - d20 Fantasy Adventures (Necromancer Games)I think this may have been the old thread where they were being tracked (prices jumping up at the time)
D20 CollectingI thought there was a list somewhere with the harder to find ones.
Complete guesses here:
FGG3 Ursined, Sealed, and Delivered
G4 Crystal Skull (I remember this being hard to find)
Lost City of Barakus
Vault of Larinn Karr
Hall of the Rainbow Mage (I thought this was hard to find, but according to NK it's going cheap)
edit: maybe divide by 2 instead of 3. Not sure on their prices at all. If you have a Crystal Skull I think it's got to be worth more than $17 (US).