The Buildin pricing model is based on a tiered licensing scheme with quotas for:

We intend to keep the Buildin tool free for occasional/light use, regardless of its purpose (personal/commercial/educational/etc).
The entire feature and capability set will be available in the free licensing tier, however in some cases (for example the build performance trending or build profile detection currently on the wish list) the performance and/or functionality may be below expectations due to the limited number of data points imposed by the tier quotas.
The free licensing tier quotas may change occasionally depending on the tier activity - in order to keep the costs for supporting the tier under control.

We're still working on the implementation details of the licensing scheme and pricing structure for heavier personal, commercial and other use.