Could be time zones. I recall reading an article about Mechanical Turk that suggested the highest-paying tasks get done the quickest. I wonder if those are often posted on US hours, leaving lower paid tasks for Indian daylight hours.
To imply that Costa Rica's education is somehow superior to India's is misrepresenting the situation.
If in my local market 300 dollars is a pretty good average then I don't have leverage to ask for 800 dollars from the HR department of a billion dollar beast of a company who will create every possible pressure to adjust the offer according to my local market.
But that doesn't explain the incentive for Amazon to hire anyone from Costa Rica at 3x higher salary? If education/skills are the same why not just hire more Indians (or ppl from some other, cheaper Latin America country) and save a lot of money? Hell, they could easily open a center in many parts of Eastern Europe and still hire people for far less than those $800.
I think the real answer is: The difference between $300 and $800 per worker for a program like this is _couch money_ for a company like Amazon. The value they will eventually derive from the data and profits is well worth whatever time savings they get from hiring someone in the western hemisphere I imagine.