Another diver here (and sailor as well). There is a certain minimum cost, but it's not as much as most people imagine it will be. I could fly to the Red Sea and dive for a week (including gear rental, cheap'n'cheerful accommodation and going out in the evenings) for about GBP 600. If you spend more money you could stay in a nicer hotel and eat imported food (if you really want sushi in Egypt it'll cost you!) and buy all your own gear but the diving won't be any better.

I'm not "rich" by any means, but I run out of vacation time well before I run out of money to dive...