The kicker for me on purchasing a rifle would be do I want to reload or not. If you don't want to reload go with .223 Rem, the cheapest thing out there for volume shooting in a center fire rifle cartridge. If you reload I'd look at the Ruger or CZ in .22 Hornet, with some of the new lighweight varmint bullets out there for a .22 cal I think owning one would be a blast.
I'm seriously considering getting a .22 Hornet in the next year. Right now my budget is kind of tied up on a hunting trip. All of my .223 rifles have too fast of a twist to use the .36 grain Berger of Barnes Varmint Gernades.