I know folks who clear over a grand a month with MTurk. I personally can make my rent in a month. It depends completely on how much time you're willing to devote to it, and what kind of HITS you enjoy working on. It's absolute best feature is its flexibility as far as when you want to work and what work you want to do. In addition to that, there is the option to have your earnings deposited directly into your bank account. Earnings are not just made available for Amazon purchases.
It's definitely worth looking into. There are a lot of very low paying hits that require little effort and concentration. I like those when my brain feels like mush - I still feel semi-productive, but I don't have to work really hard to be that way. :D