Is that because you have loads of users? (curious CircleCI employee here)
30 users + 500 builds.
However I don't know what counts as a build, since a typical commit to an open PR uses 10 GH runner machines simultaneously doing odd jobs like integration tests, releases, deploys, etc...
Pricing should mostly just be users + build minutes (for cloud runners) + storage. There is a few other optional, feature specific costs. Self hosted runners are free, but you need to self host caches/workspaces - our native ones have an egress bill to self hosted runners.
If self-hosted runners are free that would change our equation a bit. I'll talk to some folks here, I liked using this product at another company I worked at - but this would most likely shake out AFTER Github charges us the first time.