Hi there, in case you missed it on our website:
Ultralight is free for non-commercial use, educational use, and also free for commercial use by small indie developers making less than US$100,000 a year. You can find full terms in the SDK. Pricing plans for larger commercial projects will be announced later. For more information, email firstname.lastname@example.org
As mentioned, we're still working on the commercial licenses for larger teams. Feel free to shoot me an email to start a discussion.