In our market, it's at least 1:10 (CentOS is ten times as popular as RHEL). We don't actually record a difference between them in our license system (they both register as generic "redhat" with a major version), but we distinguish between them in our ticket tracker.
We had a bug in our installer recently where it wouldn't work at all on RHEL (I stopped testing on RHEL a few years ago because compatibility was so reliable that it never behaved any differently from CentOS). It took almost 24 hours to get a bug report about it (we see about a hundred new installations a day). So...it may be even less than 1/10th.
That said, we mostly operate in the low-end web hosting market: solo web developers, small design shops, web hosts selling to solo web developers, small businesses, etc. Our software rarely ends up in huge enterprise deployments. Even for our customers that are big businesses that use RHEL on the backend, they might have CentOS on their web server, because it's just a rental and that's what their web host installed for them. So, our numbers are certainly skewed toward CentOS because margins in web hosting are razor thin.
