Just from my rudimentary math skills, I'm not sure the pay-per-achievement assignment would work. As an example, let's say I have 10,000 active users per month, and from their activity, they achieve 10 things per month. At one penny per, I would be shelling out $1000 per month for your service, $12,000 per year. Simply put, hell no. Perhaps, a better pricing plan would be in tiers with fixed costs, and notifications if one were nearing the limit of their chosen tier?

Also, for that $1000 alone, I could probably hire a developer to build an achievement system on my own server for my own sites. Just curious, what would the incentive be for me as a site owner to pay for your service as opposed to building my own?