It's not necessarily that expensive - at least I'd say it's within 50%. At Oxford where I did 1:1, I believe the total time with the tutors was around 5 hours per week and each tutor was probably investing 2 hours (max) per student. That would mean a tutor could handle let's say 15 students on a full load. This is about the same ratio as a teacher at a well-resourced school.