If you're simply looking for cheap and fun to shoot, go for 10/22. .22 ammo is about $20 for 500 shots. Ar's, AK's, and the Mini-14 all shoot .223, which is more like 25-50 cents per shot on the cheap end. Is there a particular look you're going for? Because there are a ton of body kits for the 10/22 which can make it look like a tactical weapon. Take a look at MidwayUSA.com and see what I mean.

Don't get me wrong, I enjoy shooting an AR as much as the next guy, but when you take a look at what you shot that day, and how much you spent on ammo, it hurts a lot less when you realize that you could have shot all day for that $20, versus just a few magazines.

Another option would be a rifle that shoots a pistol caliber, which can be a little cheaper, but for sheer bang for the buck, the 10/22 can't be beat...plus, you can get one at your local wal-mart (Or at least I can...) for less than $300.