If I'll ever dive the GBR, the 1.5k$ airfare will be my least concern... The 3-5k, what a liveaboard costs cuts the wallet much deeper.
Anyway, there aren't many equipment intensive sports around, that cost less than 1k a year. What about ski/snowboard? About the same gear, clothing and daypass costs...
I have other hobbies, and scuba is not the most expensive equipment-wise:
#1: photography: I have 2 cameras and a few lenses. Worth: 2+k$. Travel costs adds up exactly like diving.
#2: scuba: I have a full gear except main tank and weights, costed around 1k$
#3: biking: my bike was ~500$, which is quite a cheap one...
I agree, you can throw an infinite amount of money in the Ocean, but normal, recreational scuba is not luxury. Special things, like TEC diving, expensive liveaboards, pro underwater photography are expensive. But so is a photo-safari in Africa/south America.