I've done this. I set up a survey on SurveyMonkey and then created a series of tasks on MT to send people to it.

I paid about USD$0.05 per survey and spent something like $30 or $40 (I don't recall exactly). I got thousands of people taking the survey, they were from all over the world, all different demographics, and the information was solid -- i.e., people didn't just type "jfalksdfjkjhgkj" to try to steal the money.

One thing to be aware of: since this will be an international audience, their English reading skills will be at various levels. Make sure your directions / questions are very clear and simple.