I think the relative supply and demand are going to determine prices here, just like any other market. A price floor could be imposed, but this would likely price some work out of the market or lead to other work-arounds as trading partners try to avoid constraints.

As far as protecting workers, in a survey I ran last year, there wasn't any evidence that workers on MTurk feel more exploited by their online bosses compared to their offline bosses:

http://www.technologyreview.com/blog/arxiv/24646/

Though obviously those that feel particularly mistreated could be selecting out of the market.