For an "in barn" lesson with my coach I pay $85 for 45 minutes. In a clinic it's about $175 for 45 minutes.
Another clinician I bring in is about $135 for 45 minutes.
Other people I've ridden with are up to $350 for 45 minutes.
I currently pay £12 for 30 mins with the resident instructor on the yard, she is teaching me and Reeco to longrein and is very good. I have in the past paid £85 for a 30 min lesson on a GP dressage horse at a top yard with a top instructor and thought that it was worth every penny.