Expensive? I guess you have to put it into perspective. I have as much money tied up in my golf clubs or skis as I do my scuba equipment. And I have pretty nice scuba equipment. But that in mind, none of those are inexpensive endevours. As far as a career in the sport, it is possible, but often can be tough. It all depends upon the route you want to go. Being a scuba instructor can be a great 2nd job. It's fun and rewarding and gives the chance to meet new friends.
As a full-time job, most dive shops around the world don't pay very well for shop help. It can be quite difficult to make it on just that pay alone. Combined with money earned from classes in a busy shop, though, it's more than possible.