I wouldn't say a flat-rate is exactly correct. Some sites receive tens of thousands of comments on a single article. Others, 5 or 10. The overhead on the service is more likely a better determinant, such as bandwidth and performance requirements for hosting. If SiteA uses 95% of your traffic and SiteB uses 5%, and your overall costs are 200 FakeUnits, you ought to be charging in respect to usage. Starting it at a price that wouldn't impact you negatively would be a good idea, and making it scalable would be even best. I'm moreover just throwing out ideas and thoughts though, I'm no VC or business expert myself, just someone who hosts servers for fun.

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At this point I'm not making anything and I have poking at the idea on the side for 3 years. I'm currently collecting interest by allowing people to register for the beta.