1) If you have an interest in a certain title, tracking sales on ebay, audiogon and other sites should probably give you an idea of what the going price is. Sonically, I have found some of the MFSL cds worth the premium I paid and others not.
2) When talking MFSL Cds I think you have to distinguish between looking to buy sealed copies or used copies. In my view the prices being ask for by many for sealed copies are crazy. I mean unlike LPs where condition makes a big sonic difference, with CDs it is for the most part, go or no go, it either plays or it doesn't. If it plays, then sonically the used copy is as good as the sealed copy imho.
3) Because MFSL did not always have a CD version and LP version of an individual title and, a least from my experience, certain MFSL titles show up on the market less frequently than many others (Steppenwolf s/t for example)some premium based on supply is probably justified (see early comment about premium for sonic quality).