It's a hobby - I know people who do it fulltime and it's possible yet hard and insecure, but if you have several slots a week it's possible...certainly it's not possible to hold down a job and be a fulltime DJ also...unless you only do weekends; even then, I think it's one of those either/or situations.

Costly? It can be, although I tend to do friend's nights, and have been paid in the past, although trips to SF and Trier either weren't paid or was drinking money.