Many people think that Colorado's official state motto is "We're just a plane flight away from great scuba!" That is a myth, but it does reflect reality. Colorado is every year ranked among the tops in the United States in the number of divers per capita, so those divers must be going somewhere.
Probably the most popular dive travel site for us is Cozumel. We can usually get pretty cheap flights, and the lodging and diving there is very reasonable. I once heard someone suggest that we should try to make Cozumel a state park.
The rest of the Yucatan peninsula is also very easy to get to and very reasonable in price. My most recent trips have been to Akumal, were there is reasonable diving in the nearby ocean (especially for beginners) and easy access to the vast cave system for the more advanced divers.
If you shop around, you can get very good deals on flights to Florida. My wife and I go there for a couple months every winter, and this year my son and his family joined us there for a week using astonishingly cheap flights--I am talking about $100 each round trip.