It really depends on its usability and popularity. A foil Masticore or Port will probably set you back $40-50, over double there value. I see foil Multanis regularly around $25 on ebay (1 even at $35). I would also bet the Dragon Legends go for around $20-25.
Junk foils you can easily find for $5
I do like Azreal's rule as a guideline though, you might be able to squeeze $15 for Molimo b/c he is a legend, which for some reason seem to go for more. Probably b/c of their higher casual value.