Most lesson barns that I've been to have an instructor teach students on the school horses. The instructor gets paid hourly or per student. Per student works best, that way the instructor makes more money when there are more students, and you don't end up paying extra money when there aren't as many students. I would suggest paying $10/student and you keeping the other $20. After all, you are paying the feed, bedding, labour, shoeing, vetting, fencing, insurance, equipment, general marketing, etc.
As for trailering in, I would say $10/horse. You could offer a punch card (ex. $90 for 10 rides) or a monthly pass.
Make sure the instructor has insurance (or is covered by yours) and that all people riding on your property have insurance. Also post some basic rules. You could include no other riders using the arena during a lesson, all riders must wear helmets, clean manure out the arena after you ride (especially for trailer-ins!), horses coming in must be up to date on vaccinations, must have negative coggins, etc.