Not a huge difference between buying the root cert itself and getting a cross-sign. LE started out with cross-signs from Identrust.
"On October 19, 2015, the intermediate certificates became cross-signed by IdenTrust, causing all certificates issued by Let's Encrypt to be trusted by all major browsers."
"On October 19, 2015, the intermediate certificates became cross-signed by IdenTrust, causing all certificates issued by Let's Encrypt to be trusted by all major browsers."