I've talked a lot about the business of arts admin, but I'm also a violinist so I'll talk a little about the business of freelancing.
First of all, if you want work you have to network. Just like any business, it's all about who you know. You don't have to be an amazing player, but you have to be a solidly good player. And you should be nice (especially if you want me to hire you for anything). And join the union, that helps a lot.
You may play all kinds of gigs: sitting in for a traveling Broadway show, playing in a part-time regional symphony, weddings, private corporate events, churches churches and churches. The nice thing about freelancing is you can pick and choose what/when/where you want to play. The tough thing about freelancing is that it's not super reliable.
I've played gigs for all kinds of rates. A nice perk of being in the union is that you have the ability to play in union orchestras and events, so you get good rates for those. When I am asked to form my own groups, I charge (per person) $150 for the first hour, and $50 per each additional hour or part thereof. So if you have a wedding, we will usually be on site for more than 1 hour so you're looking at $800 for a quartet. I don't keep any extra for myself, cause I'm not running a gigging service. Maybe that will change some day. These prices are more than some of my friends charge, and less than I want to charge. Think of if this way: how much would it cost you to get four plumbers to show up to your house for two hours on a Sunday? I'd say you're getting a pretty good deal with four musicians who have 80 years of combined training and experience.
If you have any questions, just ask!