Prices of the MAK-90 have been going up lately, not only because of obama-panic but because there are no more coming into this country and more people are getting into the hobby.
Lately stamped receiver versions in very good to excellent condition have been selling for between $500 and $600 for the rifle only, no box and only one mag. New in box with accessories have been selling for $700 to $750.
Milled receiver models are selling for $800 rifle only to around $1000 for new in box.
Check Gunbroker for current sold prices. That should give you an idea as to what people are actually spending on them.
From what I've seen, and YMMV, between $600 and $1200.
Your OP asks way too vague of a question. What is the condition of the metal, the wood, the barrel, the moving parts assembly, the gas port? What's the aproximate round count? Does it come with original accessories? Has it been 're-fanged,' does it have a Bush-hole stock, or has it been converted? There are a lot of variables when it comes to pricing a MAK-90 (and many other AK pattern rifles, for that matter...).