How do you determine the amount you want to pay users for completing an offer?
Like everything else in our capitalist society -- supply & demand and market values. )
Just look around at what other people are paying, and offer something similar. Back before this influx of new people, when most of us had done all the popular sites and were either experimenting with new or fringe sites, or were proxying trades with friends and family, rates went kind of high. After all you new guys showed up, there was lots of competition, lots of demand, which drove prices down to more reasonable levels.
It will also depend on how close you are to finishing -- if you're only 1 green away from a laptop, you'll be willing to pay more for that green.
The economics of freebie trading are about as dynamic as the stock market, so you just try find the happy medium between what it's worth to you and what others are looking to pay.