I have been there... twice, and it was a great dive. I went with my local dive shop/club. I think you have to be part of a group. The time thing is not that bad. You dive for about an hour, get out, watch the show, have a decompression hotdog, get back in for another hour. There are plenty of cool things to see. Because of all the rain we had this year the springs are really flowing hard, so you probably won't get down into the cavern. If you want to chance it, there is a rope you can use to pull yourself down with. But if you slip it is a ballastic shot up until you can grab onto something else. Not to mention your reg will freeflow/purge from the pressure. Then again, it may die own some by August.
I don't know if you are interested in tank diving, but the
Florida Aquarium has a shark tank dive that is interesting. It costs around $150, so it is pricey. You are not allowed to touch the animals, much to the chagrin of the resident green sea turtle who was a performance artist in a former life. But the sharks come within an inch of your nose. Actually, one brushed up against me. The handlers do a real good job keeping them away though, and they only allow the sharks to come close if you seem comfortable. The only thing you need is a C-Card, and optionally a mask. They will provide everything else, and it is all Zeagle equipment. Overall an enlightening dive. Now I just need to try the Bahamas shark feeding experience (this Spring I hope).
There are also some nice Gulf dives off Tampa. You have several holes and quite a few wrecks. I don't have my book with me, but I seem to recall there being an artificial reef made from submerged Army tanks in the area. I know it is somewhere on the west coast. Speaking of that book, Ned Deloach put together a great book called "Diving Guide to Underwater Florida." I am in France at the moment, and my copy is back home, otherwise I would post the list of Tampa sites. The book is less than $15, you may want to just order it (I think Amazon stocks it). It is an indispensable field guide to Florida dive sites. Also, you may want to post this question to the "Florida Conch Divers" forum.
Alright, so there is a dollar answer to a nickel question.