money is hard. which is why people switch to electro house and media instead of music.
the sources of revenue i've had any first hand experience with are direct royalties, remix fees, library music fees and selling instrumentals.
Royalties can be decent with a decent label. The sales need to be there, that's about the thick of it - digital download I've earned probably £30 a track through other labels, which is pretty low for the time spent. CD compilations about £50 a track but they wanted exclusives. the more material you can get onto a compilation, the better. that's why it's good to know the right people. when they're putting something together, they'll come to you. Incidentally, if a new label message you and immediately start talking about royalty agreements and you a) havn't heard of them and b) don't have any decent artists on their label, don't confuse "royalty agreements" with "actual money."
remix fees come from your reputation and you need to be a name that will sell to get them in the first place, I used to charge £50 for a remix, probably more like £100 now if I was asked. I know some bigger artists can easily charge £1000, I dread to think what John B or High Contrast would charge. That was what my mate paid for a remix by Alphazone, same by Pervading Call but he never took it on.
Library music, it's worth putting everything you write on a lot of sites and waiting for the cash to come in, which can even take years. Tagging is very important. For tracks I didn't write for library purposes, I've probably earned £50 total without really doing anything from productiontrax. It's not a lot, but it's as good as free because it's making the most of stuff you've already written.
I know people who offer studio days and again, once you're established, you can make some decent money from them. With good kit you can be looking at £80-£100 a day for engineering. But again, it's reputation and popularity. People don't just want your abilities, they want you to bring them up from the low rungs and make them a name. And that's what you have to do. I would say an absolute majority of tracks written at studio days have either a) already been signed before they've been written or b) get signed to a label that's something to do with the engineer.
Selling hiphop beats was pretty easy. I did mine mostly through...I forget the site. But that often earned £30 a track, and with a good workflow you can knock a simple track together in a few hours, add some loudness on the master bus, done. Non exclusive rights worked well for me.
You can probably ignore things like youtube and adwords revenue too. Only places like UKF are going to be making anything even slightly considerable from advertising.
Gigging, I've never earned any money. But again - reputation. I'm not a known DJ at all. I'm sure even mid range DJ's earn a decent wedge! I hate travelling.
All I can say is, personal experience has taught me that you need to do things for free, and they will lead to things that are paid. Engineer for people, collaborate with people, send plenty of promo's out to DJ's and labels. People don't want to work with you if you havn't done anything for them.
That's about all I know, hope it's of use to somebody. I may be right or wrong about things, but that's just my experience of it all.
don't quit the day job!