Unfortunately, it's not a quick question. Fortunately, it's a great question.
How much do you need to buy?
A lot, actually.
Well, depending on your version, you'll most likely need the Player Handbook, Dungeon Master Guide, and Monster Manual. Those three are really just for the Dungeon Maaster/Game Master, and the players can get away with just the Player Handbook.
I got the 3.5 version, my particular favorite, and those ran me together around $100.
Another advantage of 3.5, the players might be able to get away without getting the handbook, either, and just using http://www.d20srd.com
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On top of that, there are the dice. A polyhedral set can run between $10-$20, depending what you get. Doesn't hurt to spend a little extra to get some cool and not Chessex.
And as a DM, you'd usually want some customizable or drawable grid to stage the fights and encouters. I made it cheap by sharpie-ing a grid over an old whiteboard, but I'm going to have to find out how to make it better. I got away without getting miniatures by laminating 1"x1" squares which I drew with various pictures.
After that, though, it's up to the DM to make the best campaign he or she can.
It is a lot of work and there's a decent upfront cost for the DM, but as a new DM, I can say it's totally worth it.
If you're interested in D&D, though, I highly recommend joining one of the campaigns going on here. I'm just starting a light-weight 3.5e-style D&D campaign set around zombie survival that might be a nice introducer, or it's possible one of the more experience (and infinitely more awesome) DM's can fit you in the party.
There's Redemption of Agronon above, DM'd by fathom, which you can read through, and there is the Roll20 campaign that Arctic leads, that once we start it back up, you could sit through and watch a campaign or something.
So, I'm super stoked that you're interested in D&D. We could always help provide links for resources, but if you're up for it, try joining a game to see how it goes, especially if you're going to be the DM.
I guess, first question, do you know which of your friends will DM?
And yes, that's an awesome sig quote.