It sounds like you have 2 choices. Either trade for a gun with open sights or figure out how to get a scope.

I won't try to tell you how much to spend on a scope, but I also won't sympathize with you when buy a cheap one and it craps out. You should buy the best optics that you can afford, and if that is a used $25 cheapo, you can always upgrade as you can afford it.

I have a box in storage with some old scopes that came off guns that I have bought over the years, and I will see if I might have one you CAN afford.