IIRC, it was on the order of $100k+ per cluster for the Premium offering. Has been several months so I would suggest contacting them to see what it currently is.

For some projects I've been on, that wouldn't be a big deal. For smaller projects it can easily be several times your opex though.

You can run an open source cluster in HA without it though. The big draw for Enterprise is the multi-cluster and HSM support. Anything HSM is typically quite expensive so their pricing really isn't out of line.