Cirrus CI is free for Open Source projects. For private projects Cirrus CI charges a monthly fee of $10 per seat for orchestrating builds on supported computing services.
Here is a pricing model of Cirrus CI:
|User||Public Repository||Private Repository|
|Person||Free + Access to Community Cluster||$10/month + Access to Community Cluster|
|Organization||Free + Access to Community Cluster||$10/seat/month|
See all the pricing options on Cirrus CI's GitHub Marketplace Page.