How do I keep a margin for my own site?

Every amount you see is what you earn. How much of it you pass on to your users is set by Units per USD earned on your placement.

An example: in your app, 1,000 points are worth $1, and you want to keep 30%. Set Units per USD earned to 700. A completion that earns you $10 then gives your user 7,000 points, and you keep the rest.

Your placement settings preview what real offers will show at the rate you type, before you save.