This.
Not to mention, the kick backs aren’t a commission. Trainers are required to sell X amount of NEW and used saddles in their barn per year in order to get a free (if you’re a top top trainer) or trainer discounted saddle. They do get discounts on tack, free hats, etc. However, when I was leaving 6+ years ago - they were cracking down on trainer contracts and pulling back a lot of the Oprah style giveaways.
So yes - your trainer probably unfortunately thought you could afford such a saddle and maybe you’re a hard fit, as a lot of males are, which means the used market was small for that. However, I’m going to assume they put you in “pro” or ST panels, which you can usually make work on most horses unless they are oddly shaped. Plus, I wouldn’t get a custom horse-fitted saddle made for a 4 month lease…
Anyway. Those prices are pretty on par with a top show barn, and you did agree to pay vet and farrier. While, I agree, maybe those bi-annual and annual costs should be at least split, if your contract doesn’t say anything about it… you’re stuck with it. Which is a crappy lesson to learn, and I’m sorry you’re having to learn it that way.
Next time you lease- spell everything out, down to what “kind” of vet care. All my lease contracts had specifications of what I was responsible for vs the owner when it came to care, farrier, vet, insurance, etc. Also - let’s keep in mind you didn’t pay an upfront lease fee. While the horse is 25, he could be an excellent teacher and quality animal, and you got away with not having to pay that fee. Heck, a lot of 2 week show leases with top barns are more than what you’re paying in 4 months of board and lessons alone.