The price probably will be more than 10k, and you will sign an official contract with that root CA.
P.S. I also believe those might only grant you a intermediate CA without allowing you to issue other CA certificates under.
To even be considered for a public intermediate or cross-sign, you’d also have to pass a WebTrust audit at the absolute bare minimum, which is tens of thousands more dollars (and potentially tens to hundreds of thousands for new security equipment and hires).