From the site:
"Q: Where do my monetary stakes go?
A: To the GymPacters who made it to the gym! Every week, we will pay out cash rewards to those who met their Pact from the Stakes paid by people who didnâ??t quite make it. Rewards can be cashed out like via Paypal. We think if you put real money on the line, you should get real rewards."
Seems they don't pay out per session: you get a set amount for meeting your target (they don't say whether it's weekly, monthly etc.) . I was being generous when I said less than a dollar- probably more like cents.
They charge a minimum of $5 per missed session, so just forgetting to log in on one day could take all of your rewards away in one go.
Only way to find out is for someone to try it...