I think you need to set your boarding fee taking into consideration your own costs plus the going rate for similar facilities.
When people say you don’t make money on boarding, they are taking land cost, either mortgage or lease, into account. Obviously if you just take feed and labor into account, then you are taking on a profit and if your farm was fully paid off, been in the family three generations, that might be an income stream.
What is it you want the trainer to do? Is it just the trainer with 5 horses and you with your own horse? I myself wouldn’t see much point on having a trainer at my private barn unless they were working with my horses. The advantage would be getting a great in-house coach.
Do you want the trainer to be a draw for more boarders and students? If so does this young trainer have enough draw to pull them in?
Honestly your young trainer paying full retail for board is not going to be making much of a profit herself on say two lessons horses and two resale projects. If you do the math, say a $3000 OTB that takes a year to turn into a lower level jumper will cost her over $8000 just for board at a modest $700 per month. Not including farrier and vet. If she can sell him for $15,000 she might break even or make a couple thousand clear profit. If you want 10 per cent of her sales, she is losing money.
Your young trainer is not necessarily realistic about her own business plan. There is no way a young trainer with 5 horses in full board is going to make that much money on her program. She won’t have enough horses or assistants to run a back to back lesson factory. She won’t have space for many client horses. She won’t t be turning $3000 OTTB into $50,000 horses. People won’t pay that kind of money for a horse from a tiny operation.
She will not be a cash cow ATM for you . More likely in six months you will be negotiating a dry stall rental for her or she will sell up and go back to working on a pub or back to college.