I understand your concern. At this point, we have no plans to charge for Pathfinder. Furthermore, if you're concerned about future viability of the API, all of our code is MIT-licensed at https://github.com/csse497 so it can be self hosted.
What is your plan for long term profitability if you have no plans to charge for this? I am nervous building tech around a free service that may either transition to a deal-breaking price (google app engine) or may disappear entirely due to the business going under.
This would be an API I would build my business around. No way I would touch it without an idea of how much it will cost me once it exits beta.