My feeling is you should get more than an hour's use for 1 hour of training. You are putting a lot of skill and work into training a young horse, and the owner's arena will be there whether it's used or not. Look at it in monetary terms....how much would she have to pay for training? $20 an hour? $30 an hour? And how much would you have to pay per hour to use an arena? One of my clients has a private indoor arena, and lets a couple friends use it for $5/hour. Maybe you could check and see what the going rate is in your area. Don't sell yourself short!!