As someone who has worked on a freelancing app, I can tell you that getting your hands on this data is not as easy you'd think. Problem is that many freelancers don't show up in traditional employment statistics, or if they do the data is sparse. So you're left with a lot of ballparks that don't really tell the whole story. Expensive consulting reports are your best bet.
I imagine this will change soon, though, as the gig economy is exploding and all eyes are on companies like Uber and Airbnb, so data will start surfacing. Though that doesn't help people who want to get an edge right this instant.