Sorry for the late reply. Shure, it is hard to develop a good pricing model, but if your approach is serious you try to figure it out pre-launch, so that a potential customer does NOT have to think about how much he would like to pay for the service. He either accepts the offer or he doesn't, and a tiered model (dependant on your use) should offer a good deal for everyone. I personally would argue that twice the free tier would be worth 10 bucks a month, double that 15, and double that again 25. This way would reward bigger users with a volume discount, but still make the service accessible for more serious hobbyists.