Something special to look for on a stamped one, is the rear of the receiver is cut at an angle on some and straight on others. The slant-cut ones are less desirable. If it has the factory thumbhole stock on it you may have to remove the stock to find out- its harder to tell from just looking inside the receiver with the topcover off but that would be worth a shot.
There were some with underfolder rear trunnions, those seem to be the most desirable.
Price is whatever someone will pay. They're usually going to cost more than a current Romanian AK, but not through the roof expensive. I've seen some sell lately for $550-$600 but higher or lower depending on condition isn't uncommon.
Milled ones will normally sell for more than stamped ones.
Search Gunbroker for just Norinco. There are several MAK-90s listed, most with no bids. You might try also seaching the Norincos and then select to view the auctions ended within the last few months to see if any truly sold and for what price.
Like TREETOP said, current range is around $500 to $600 for the stamped ones, with milled for a hundred or so more.
All milled ones have a strange double slant cut to the receiver. Ironwood Designs make a special stock to fit. Some stamped ones have a slant cut to their receiver too, and as said are less desirable but Ironwood also make a stock to fit them too.
Some had threaded muzzles, most didn't. Few had the muzzle turned down so that standard AK 14mm threads don't fit but they can be threaded to 1/2x28 if desired. Very rare ones have the cleaning rod holder intact with just the bayo lugs ground off.
The Chinese made some really nice commercial AK variants both before (pre-ban) and after (post-ban) the George Bush the First stupidity on how the looks of a firearm can turn an honest person into a mass baby killer. Unfortunately william jefferson clinton put an end to Chinese imports, along with others which is a shame as Norincos had really nice triggers, thicker receivers and generally a nicer finish if you like blued firearms.
Most collectors of Chinese AKs generally think the above mentioned prices are about the norm now for the MAKs. Don't worry about whether the muzzle has the threads or not as this feature is rarely on military AKs once the Chinese went to the stamped recievers. I'm glad the milled MAK I traded from a friend has the muzzle right at 14mm which will thread up perfectly since the milled type 56s had muzzle nuts. Will make returning it to the way it should be easier.
I bought a stamped Mak-90 two weeks ago for $550 from an individual, so no tax. It is in really good condition, but is a slant cut receiver. (Noticed original date of post after I posted this), so I'm sure you've already bought it by now :-)
I would argue that straight cut MAK's are very good shooters and fetch a higher price due to their heavy barrel and inherent accuracy. If you can find one for $500 snatch it up. I would say $700 in good shape is normal. $1000 is high.
They are nice rifles with about the best trigger offered in an AK.
I have seen them anywhere from $450-$550. for stamped and $600-$750. for the milled.