Of the calibers that you mentioned, .308 is going to be the most economical. There's really no practice ammo for the other two that you mentioned. If you are shooting for no purpose other than targets and hitting the target is the only concern, there are other options to get started.
Getting a .22 was already mentioned. I know that it's a small round, but everything that makes a good shot still applies. If you have your heart set on a larger caliber, I'd go for the .308. I like the 25-06 better because it shoots flatter, but if you're on a budget, .308 is that way to go. Lots of guys shoot .308 for a number of reasons. The brass is plentiful and if you are serious about target shooting, you're going to have to start loading it yourself. .308, in my experience, is very accurate too. The slower moving projectile is also going to be easier on your barrel. Quite frankly, if I had to choose one rifle (larger than a .22) and could only have that one, it would probably be a .308. Very manageable recoil and everyone carries it.