For me its one of the cheaper hobbies I have. Im rebuilding a XC falcon and rebuilding cars is never cheap, nor is photography which is another hobby of mine. Gaming is another thing I do which, again, isn't all that cheap which just leaves aquariums and skateboarding. Once setup its only electricity, water and food for the fish and skateboarding just requires a board so that's cheap as.
As for why I keep fish, not only do I find the process of fish keeping (feeding, maintenance, setup etc) both enjoyable and relaxing I like the way the tanks look in my house and find the fish I keep interesting. I also enjoy reading and discussing about this hobby which varies so much from person to person and while automotive has the same ability to get you shunned by discussing it with others who don't enjoy it the variety in method is quite the same.