If your goal is to build 'a' deck, then it would be cheapest to do it through singles.
Research, digital theory craft, then buy your deck list.
Building a collection, or wanting to make multiple decks: Booster packs, in a box form. Typically, two full boxes will get you a play set of commons and uncommons, and a smattering of the rares and mythic rares. I only bought a Fat pack from BFZ because of the lands, though only because the LGS near by sold it at MSRP.
I've got a wealth of cards in binders, because I like being able to build a vast variety of decks at the same time, dancing all over the color wheel.