About 4 or 5 years ago I was doing freelance TW instruction for 35/hr cash. I'd call a local flight school and see what their rates are. I think instruction is in the ballpark of $50/hr these days. Some of which the house will keep, so interpret that as you will. You may want to just charge that, or maybe less since it's cash. The latter is what I was doing back then. Instructors were taking about 17-20/hr before tax. I charged less than a flight school would but kept all of it.