For a really cheap, small small game option, then a Marlin Model 60 .22lr. About $125 for the rifle, and 500 rounds for $20. Good for rabbits, squirrels, prairie dogs, etc etc out to about 100yrds.
If you're looking for a little longer range from a centerfire cartridge, then Stevens Model 200 .223rem is a great way to go. Last I looked they were about $300 around here, and a box of hunting quality ammo should run about $17 for 20rnds. My personal recommendation would be to save up a lil extra money and opt for a Savage 12FV for about $550 with the Savage Accu-Trigger... AMAZING shooting rifles...