New PS90's are selling for around $1,500 but it really depends on where you buy.
Anything more than $1,700 and you're paying too much.
Used ones are selling from between $1,300-$1,500 depending on how old and condition. If you are going to buy a used one then look for one of the newer ones with the black reticle sight. The older ones came with a white reticle sight and the reticle is notoriously hard to see in bright light.
I would suggest buying the version that comes with the optical sight and not the version with the rail. Both versions are priced the same but the optical sight is a lot more expensive item by itself than the rail. So if you buy the version with the optical sight you can buy the rail and swap it out. If you wanted you could even sell the optical sight and probably get a few hundred dollars for it on gunbroker pretty easily.
I got my used PS90 back in 2009 for $1350 and it was LNIB.