Subsonic, probably about 18-25 cents a shot.
Supersonic, probably about 35-45 cents a shot.

With that said . . . it is extremely hard to estimate accurately because of the vast array of bullets that can be used. Virtually any bullet that measures .308-.309. And obviously, cast bullets work just fine for subsonics, so those could be "free" (except for your labor cost in casting them). For supersonics . . . the sky is the limit, but "basic" jacketed bullets are not very expensive at all.