I suggest you consider Ft. Lauderdale if you're looking for a reasonably priced short dive trip. The reefs from Ft. Lauderdale all the way north to Jupiter are excellent, and if you prefer wreck diving there is plenty of that to be had too. Also, you're close enough to the Keys that you can easily drive down and stay there for a night or two if you want. You didn't mention what time-frame you have in mind in your post, but if you can wait until after mid April, the prices in Florida come down significantly.
What makes south Florida an ideal destination for short dive trips is that you can usually get low cost airfares. If you search Expedia or Travelocity, you will often find roundtrip airfare from MCI to FLL in the range of $200 to $250 per person. You can't get anywhere with decent warm water diving cheaper than that. As for hotels, if you're willing to stay a couple of miles off the beach at someplace like a Ramada, Days Inn or Red Roof, you should be able to find a room in the $60 - $80 per night range. Finally, you will need a rental car. With a little shopping you should be able to find a midsize for no more than $35 per day (excluding insurance).
The prices I'm suggesting all assume traveling in off-peak times. To get the best airfares, you probably need to stay-over a Saturday night.
If you want more information about dive sites or dive operators, check out the Florida Conch divers section of Scubaboard.