How do I pay for SCUBA? MasterCard of course... it's the American way!
Seriously, I'm one of the fortunate ones who has developed a (largely unearned) reputation for being someone who knows something about marine life (but not necessarily diving per se). Thanks to that, and a PhD in marine ecology, I'm blessed with an LDS that has given me the opportunity to keep expenses minimized in exchange for sharing my knowledge with their customers.
I dive on their dive boat and get air fills for free. All I do is help customers identify and learn about the marine life they see on the dive boat trips.
As for expensive dive gear, there is no way I'd buy much in the way of new gear at the MSRP. I like older equipment and buy it off E-Bay after ensuring it is in good condition. I definitely do not replace "old" equipment just because it is old. My wetsuits often last 400-600 dives, although they are tattered and torn and full of holes by the time I usually retire them. My current BCD has about 1,500 dives on it and is still looking quite good and is fully functional. Only one of my regs is newer than 10 years. I buy older ones that are rock solid (like the SP Mk10/G250) used and make sure they are checked and rebuilt if necessary.
Maintain the equipment you do get to increase its lifespan. Although the initial costs can be high, you can keep them down by making sure your equipment lasts.
I also have (some) skills in UW videography. I've occasionally been able to trade out finished product for free diving services in foreign areas I visit. For example, I did a one hour DVD in Belize and gave the finished product to an operator who let me dive for free while I was there.
I'm not interested in a real job since I'm so focused on my underwater videography and education. However, my bank has been suggesting I consider it! I'm having too much fun to go back now.