Two schools of thought here; economy, and value.
If you don't already have it; I can strongly recommend getting the book "Yeast" by Chris White and Jamil Zainasheff. In it you will see how to build your own yeast lab which will enable you to store samples from vials or smack-packs to propagate and use later, or re-use yeast from another batch, as tom_hampton does in some of his brews.
While economical, it does involve a bit of care but it's all stuff the average homebrewer should be able to do. If you can keep your beers free of infection from kettle to keg (or bottle), then you're most of the way there already.
As far as value, you're not likely to save a ton of $$ propagating your own yeast cultures but it can be fun and challenging, and is yet another part of homebrewing where one can have seemingly total control.
BUT; seeing as a vial of fresh liquid yeast costs around 6 or 7 bucks, and your grain bill's twice that, and then there's hops, in the grand scheme of things, that $6 or $7 vial or smack-pack is probably the best value on your shopping list because ultimately it is the yeast that makes the beer.
