It's not much. I don't want to mislead you and make you think that it's a lot of money. It really isn't. I've heard that if you do it long enough, then eventually you can make it a reasonable part-time job, but you have to go through a lot of low-paying tasks, like hundreds of them, and only get about a couple hundred dollars from it after way too many hours of work.
If you need a way to pay for gifts for friends and families for birthdays and the holidays, then it's a good place to save up money for those types of things, but definitely wouldn't depend on mTurk on any livable income. If you're a writer, like a good writer, who can write quickly, then sure, you can make a pretty good buck at it, but for the most part it's just data entry and repetitive tasks that pay like five to ten cents a pop.