I think it's great that they will even credit you. It's not their fault your horse can't eat the hay that every other horse in the barn eats. My horse eats special grain, and I pay full board plus supply my own grain.
$500 a month for self care per horse is MIND blowing to me. When I was at a big facility that had an indoor and outdoor and provided round bales I was pay $325/month for self care 24/7 turn out.
Now that I have moved I pay $150/month for self care which is 24/7 turn out with a run in. I am awful in the mornings so I give them an extra $25 a month to feed her with the other horses but I provide grain and hay, blanket her, etc. I get a little tack room, and there are 5 horses on 8 acres. No ring but besides that we are so happy there!
I live in one of the most expensive places in America, and even pasture board here is EXPENSIVE at $400/month! I do self care co-op right now for $235, will be $185 once I become a student of the university. That's way, way too high.
And we do credits if your horse has a medical reason for different hay/food, but it's a set, agreed upon price. IE $80/month credit.