Hunting rifle... I'm guessing. Doin' it on a bit of a budget?
If you're into the "math" of it all... about 1/2 or more
than the price of the gun is a good guide.
You don't put a $2000 scope on a $300 rifle... well, you don't unless you have that scope just "laying around".
Inversely, you don't put a $150 scope on a $2000 rifle... well, some do, but that's just wrong