If you're just looking to replace dry yeast as a money-saver, don't bother...you'll spend more $$/time culturing the commercial yeast.
Generally, when people try to culture a commercial yeast it's to get a specific flavor profile from a yeast that isn't available to homebrewers. The two examples that come to mind immediately are Rogue's Pacman yeast, and Unibroue's belgian strain (both of which, BTW, are seasonal/special offerings from Wyeast...there's also some debate as to whether the Unibroue bottle-conditioning yeast is the one they use for primary fermentation).
Make one trip to your LHBS (or order online), and stock up on several varieties of dry yeast (Nottingham, S-05, S-04). Pitch on cakes and/or harvest/wash the yeast to stretch your $$. Buy liquid yeast when there's no acceptable dry option (hefeweizens, most lagers, belgians, etc.). Wash and reuse those. And only culture the commercial stuff if (a) you can't get it any other way, or (b) you just like to tinker with things like that.