Money is empirically less effective at motivating me than magic purple pixels. Seriously, the subjective experience of being offered $X00,000 was a lot less fun than most WoW loot upgrades. Offering me $10 of my own money won't get me to go to the gym.
I quit WoW a while ago, and restarting WoW to get me to go to the gym more often sounds like it will be as effective as addressing tooth decay with an icepick. But if you can capture the magic of "Ding! Gratz!" for me when I'm at the gym, I'd pay you a substantial portion of the hmm $6k I have spent at my gym.