The Bushnell Banner is a decent scope for the money, though it may cost you a little more than $50. You can pick one up for under $70.
http://www.midwayusa.com/rewriteaproduct/294067
This site has some really nice prices on a lot of other scopes also.
I just purchased a Banner for my muzzleloader and I am happy with it so far.
Good Luck