Under what line item in your busget you put the Mechanical Turk expense? Don't you have to go through your HR department to "hire" the Turkers?My answer: I do not hire anyone. I am simply paying Amazon for some software calls, in the same way that I am paying Amazon for their EC2 service, their S3 service, and so on.
From what I know, I place some requests using software tools, and Amazon returns back the answers. I have no idea who completes the request. OK, I know that someone with the id AC5CHSQTK8O9Z completed the request but this may be an Amazon employee (temp, contractor, part-time, or full-time). That's why I pay Amazon, not the worker AC5CHSQTK8O9Z. It is the task of Amazon to send the tax forms to AC5CHSQTK8O9Z, to make sure that AC5CHSQTK8O9Z can be employed and paid, and deal with all the HR stuff.
As a parallel example, in Starbucks I know that Mary prepared my doppio espresso but I do not hire Mary as my barrista; I pay Starbucks and Starbucks pays Mary.
So, in my budget it goes under "software" and paid together with my expenses for Amazon EC2, S3, and so on. Incidentally, this is exactly the same way that I get coders on Rent-A-Coder to prepare customized software for me. In that case I pay Exhedra Solutions (the owner of Rent-A-Coder), and themselves they pay the contractors that participate in their marketplace. Again, the charge goes under "software", since what I am buying from them is actually custom-made software.