Thanks everyone for your comments! Sorry I've been absent ... on a much enjoyed vacation, finally back home today.
The situation is that I've found a wonderful woman to take care of my two drafts, and I swear she does a better job of it than I would if I had them on my own property. One has been with her for 1.5 years already, and the other just moved in (long story about that).
On the times that I have filled in for her so she could take vacation (she pays me in board reduction), I have noticed that my beefcake definitely poops more and has a much messier stall than the others. I can do all the other stalls in 10-15 minutes each, and his takes 20 each day, easily. And that's not just because I'm picky about making my own horse's stall neater than the others.
So now that both of them are with her, and Winter is here and she's noticing that the round bales are disappearing faster than when she had five light horses ... I'm the one who actually wants
to compensate her more fairly. She has so far kept saying all the things you-all said above. I was trying to see if maybe a less biased community could suggest some pricing.
I supply my own feed, so mainly the cost is in shavings, hay, and time spent on the dang stalls. I've told her that my previous barn charged an extra $25/mo without any justification or calculations going into it, just blindly tacking on $25/mo which I always paid.
So, that's been my starting point ... for February I was going to kick in an extra $50/mo for my two boys. Alternately I could keep buying her thank you gifts.
I chip in with extra help whenever I can, but it never feels like enough.