This is an interesting study about the relationship between the speed in which HITs are completed and the number of HITs available. And the reward doesn’t seem to figure in.
“Results from linear regression further show that the average speed at which HITs are solved – which is clearly directly related to the supply of online labor for a given set of tasks – seems independent from the price of individual tasks. This surprising result seems to holds under various specifications of the price variable, including dummy variables used to split the price distribution between lower-price and higher-price tasks, and also with various attempts to remove further outliers (very low average speed HIT groups, etc.).”