The fair price to pay is what the gun is worth to you.
If you want an idea of market price Go look through listings in the WTS/WTB sections of this forum (using the search may help). If you can't find recent trades there, the look at some of the online sites like gunbroker, gunsamerica, etc. and compare prices. Also consider that most of the prices you see there are guns that are not selling, so go down just a pinch for an actual "moving" sale price.