Liveaboards like Blackbeards will probably give you the most diving per dollar spent, since they're all about diving, all the time. If you want more topside adventure at the cost of some bottom time, there are lots of great options.
Bonaire is another good option for this: Unlimited shore diving (the easiest shore diving in the world, too) for VERY reasonable prices. Dive as much or as little as you want. Closest thing to a liveaboard that you'll find on land.
Cozumel and Roatan are definitely among the best values, [semi-] locally, although as paperdesk pointed out, Asian/Indonesian locations offer some of the best diving in the world for relatively small amount of money, just a larger chunk up front. (expensive to fly there, but live like a king once you're there)
This is a good thread to read about doing Cozumel cheaply, on your own, if you're willing to deal with the relatively bare bones trip: in return for LOTS of diving.
"Coz on a budget" Trip Report - Feb. 2009