You won't be able to find anything cheap that can get the job done. Probably the cheapest would be a .25 Benjamin marauder but you're looking at headshots only and ideally 35 yards max for a clean kill. Another alternative might be the .30 Hatsan BT65. Lots of people complain about the drop-off of .357 and bigger. Probably the cheapest you could go is like $500+.