If you're joining an established band, a lot of that also has to do with how much you bring to the table. I get a "full share" as bass player, some lead and backing vox, some keys, and I produce the demos, do cover art, promotion, set up the PA, etc. Our backup singers don't get quite as much, but they don't load/unload, play keys or guitar or whatever. As the rookie (greenhorn?), don't expect a full share the first few time out until the band realizes your full value. The guy who owns the PA or other equipment also generally gets more for their trouble and investment--again, bringing more to the table. If you do the Geddy-style double-neck and foot-pedal routine while singing lead, you'll probably get the full share right off. Good harmony backing vocals will go a very long way.
Also, the better your rep as a band and the more people you can bring to your shows means you can charge more money for the show, and each make more. Again, unless you're joining an established band, you'll be starting at the bottom, but if your show is good (a good draw), then you'll work your way up. As well, smaller venues know they are "entry level" clubs, not willing to pay a whole lot for a band, but a good place to start. A few good shows there and word will get around.
If you cover your expenses the first few times out, you're doing good. But you can get to the point where you're making some extra coin pretty quickly if you work hard.