Here is an article [1] trying to estimate that kind of numbers (wage, number of active workers, etc.) from various sources and questionning the ethics of using MTurk for science purposes.
The bottom line is: one third of tasks are achieved by people relying on MTurk (as primary or secondary money input) having working conditions that would not be considered acceptable in any first-world country (hourly wages around $2, working long hours with, obviously, no kind of worker advantage or protection).
The bottom line is: one third of tasks are achieved by people relying on MTurk (as primary or secondary money input) having working conditions that would not be considered acceptable in any first-world country (hourly wages around $2, working long hours with, obviously, no kind of worker advantage or protection).
[1] https://hal.archives-ouvertes.fr/file/index/docid/569450/fil...