Ditto Panzer's statement. Around here in Upstate S.C., they are going for $650.00 to $800.00. Great if you're selling, sucks if you're buying. I bought my first one lightly used in '94 for $200.00 in the box, with 5 rd, and 2 30 rd clips, and about 100 rounds of ammo. I'm almost embarrased to say what I purchased my last one for. YMMV.
If it's an NHM 90 it will have a 16" barrel. The NHM 91 has a 20" barrel. Like an RPK. The NHM's are much nicer than a standard MAK 90 generally. Better finish, most still have threaded barrels hidden by a tube on the end of the barrel. Generally the wood is better looking. Mine is the most accurate, best finished AK I own. I keep thinking about trading it but just can't bring myself to do it.
"Better to die for something than live for nothing"
Prices vary state to state. I picked up a NIB Mak90 6 or so months ago from a private collector. I paid more than I wanted to at around $700, but the upside to my purchase was that I got to look at and pick through 9 different NIB Mak 90's and get the best one.
Having that said, for something in near unfired condition in the model you are speaking of would be a really good deal anywhere from $500-$600+
If it were unfired folks in these times will pay a higher premium, NIB Mak 90's sell on gunbroker all the time for around $700+.
And just to throw it out there since someone else brought it up, the NHM 91 I own has way better bluing than all the NIB Mak90's I have seen.