To be honest man, it is really hard. You get more from tours than CDs, but it's not easy. Not until you're huge in a mainstream way. Not a mainstream metal way, but all-the-way-radio-blow-up way. Then you get TV spots, more on clothing merch, more for advertising spots, when it's no longer you asking to do something but event coordinators asking you to do it. That's when you start making enough to make it full time. And just barely. For lesser known bands, it's impossible. I could never realistically live off my music for example. Not ever.
A ballpark figure for a "big" venue gig would net you in the 10s to 100s of thousand dollars, which you then piss away to repay the venue costs, the management, touring crew, gear costs, transportation, catering, security, etc etc etc until you get your cut, which you then have to divide among your members.