Not very helpful, but I paid around £200 for my 95+% unmolested Gold Sierra in .177" in around 1998.

And, sorry, it's not for sale.

On the description above, I'd say yours could well be worth what Mach says, but, as it's neither Northern Arms fish nor Venom fowl, it might wait around a bit on sale until the right person thinks "that's quite tasty".