Wellll, once you start, you won't want to stop. It's very interesting and uhhh, pretty cool. I like night dives for lobster.....yummm
It can be expensive. Before you start buying equipment, rent all your equipment, from what I remember it's about forty dollars a day. If you end up liking it enough, then buy the equipment. For basic setup, mask, snorkel, fins, it'll set you back $100.00 - $200.00. If you get into regulators, bc's, tanks, gauges, etc., it's like anything else, you can buy lower end, higher end, or something in between, $1200 to $5000. But once you have everything, there is not alot of expense, other than tuneups of your equipment, air for your tank ($6.00-$10.00), and cost of dive boat charters (if you don't know someone with a boat [:(]).
Good luck, have fun, be careful.