We've thought quite a bit about usage-based pricing, but found that doesn't work psychologically for most people. Generally, people seem happier by paying up front for access, then feel good about having the system available whenever they need it, rather then having to think through cost tradeoffs every time they want to do a search or use up credits. Please do reach out at [email protected] though, we'd love to talk about a solution for you and get your feedback.