For me, I'd pay up to maybe $650, maybe $800 if it looked mint too, for one with excellent regulation and shiny bores, with a 20ga on the bottom (the. I think a "fair" price is whatever you are willing to pay. I think I still got the better end of that deal. 22 POI and 5/8oz slugs 1" low, so I paid him the $400 printed on the price tag and took it home. Then he let me fire a test group and the regulation was perfect at 25yd, shot centered over. The rifle appeared only fair at first, with a little bit of surface rust here and there and some dinged up furniture, and had different than factory sights on it, so I had him talked down to $350.
22LR over 20ga Full Choke, 24" bbls soldered the whole length. I recently paid $400 even for a 24S-C in. You can't wedge the fully soldered barrels to correct it, so if you get a bad one you are out of luck. If you can, pattern the gun before you buy and make sure the regulation is relatively decent. That, imo, would be a far more versatile 24, and the shotgun shells wouldn't be so darn expensive. 22 over 20ga with a proper wooden stock, that still has the fully soldered barrels. If you keep looking, there was a few years in the 60s where they made a.
The older ones with the barrels soldered the whole length are typically regulated better than the newer ones with the separate barrels, too. They don't string as bad with repeated firing. The solid soldered barrels are a plus, if the regulation is good. If you are looking for a shooter for real use, I'd pass. It will appreciate faster than the US dollar will inflate.
If you are looking for a safe-queen as an investment that will never get shot, go for it. IMO, you've got a collector rifle there, not a shooter.