Well, there is no question about Mechanical Turk, that Mechanical Turk cannot answer, so here we go. I posted the very same question on MTurk, asking people about their average compensation per week. Without further ado, here are the results:
It is much more likely that people actually enjoy what they are doing, and getting some cash is a nice side-effect. Furthermore, it is work that can be done even while working, and doing the tasks on MTurk helps other people. My own gut feeling is that the research about the motivations that drive people to contribute to open source projects can also be applied here to explain why Turkers turk.
(Info: The current survey paid 5 cents per HIT, and received responses for 200 Turkers. I will keep running the survey to collect 1,000 responses and will report if I see any significant changes. But so far the results seem remarkably stable.)