I paid somewhere between $200 and $300 several years ago for mine. Unfortunately, it's missing the stock stud and I haven't had much luck in finding a replacement to date. Like the one G43 Collector has, mine is also marked on the top of the receiver/cylinder.
Condition is always a factor to value. These little air rifles aren't that often seen IMO. If you want it, buy it. Long after the sting of the price is gone, you'll still have the enjoyment of the rifle.
Here is my Mars115 with an interesting stock. I bought this at an Ohio Gun Collector's show in Columbus, Ohio in the late 1960's. Cost me $25.00 which was no real bargain at the time. Marked on the side like Raul's.