7.62X54R ammo has probably got the best cost & availability right now. This is if you don't mind shooting corrosive ammo and the extra steps in cleaning that need to happen fairly soon (not more than a couple hours in a damp climate) after shooting it.
For semi-autos, I see 3 main contenders, I currently have 2 of these:
SVT 'Tokarev' Pros; Decent arsenal-refinished 1940 models can be found for <$1,000.00 Not match-grade, but a very natural point & shoot rifle. Fits me well, anyway.
Can be fed with stripper-clips (Carefully! I'd gue