Getting a Cat permit
It's super easy to get on Cat. You just gotta contact the NPS office in Moab and buy one. They have a total number of users per year, and I know prior to this year they've never hit that number (I think it's 7000 per season). So, if 7000 ppl bought permit slots in May, then no one could run the river for the rest of the season. A flawed system in my opinion, but hasn't been a problem thus far.
The permit, regardless of the # of ppl, is a fixed price.