Maybe you're a billionaire or have some other source for DVDs, but if you are buying a lot I would consider Columbia House. Their standard deal is a joke and if you get referred by a friend it's even worse (as you still get the standard deal and your "friend" rapes you by not telling you the best deal). Standard deal is buy 5 DVDs for $2.50 total and have to buy 4 at full price (usually $20). If you do a google search for CH codes or just go to
www.dvdtalk.com you'll find better codes that allow you to get 5 DVDs for $2.50, 1 at $14.99 and then only 2 more at full price. This averages to about $7.18 a disc. You then cancel your account and repeat. I've been doing this for years and have built my collection quite quickly in this way.
Note that I have no affiliation with DVDtalk nor does DVDtalk in itself gain from using codes found on their site as they are "mined" directly from the CH website.