Well Karneval is still a few days off over here and the peak time will be during the five days from Thursday 12 Feb.
If you are from NCL/LBA (or anyhwere else for that matter) you will find things very strange in both Cologne and Mainz during that period (or anywhere else along the Rhine). Karneval is a fairly idiosynchratic experience.
Possibly you are even talking about a different/earlier time and hotel price hikes can be caused by trade fairs and exhibitions.
What are you planned travel dates?
This information could help throw some light on the issue and for some of us to give you better advice.