I agree with Kai...and I feel that yeast should be by far
the least costly aspect of brewing (like, almost no
cost), especially since it's so easy to maintain once you've got it.
I mainly use my house yeast (a mixed strain originally, the dominant factor being unknown and surely mutated anywy after 20+ years)....at this point I technically owe the yeast money
I've maintained it since at least the late 1980s and after re-culturing it I'll re-ptich it anywhere from 8-12 times before starting over with a new re-culture or, infrequently, a round of brews with a smack pack from which I'm also able to coax at least 8 or more brews.