I boarded at a barn where I normally had to supply my own grain, my own bedding (rarely stalled) and my horse got x # of flakes a day. So what happened is I increased the grain and I supplied alfalfa to supplement the grass hay at my own cost while she was pregnant.
I didn’t have to pay any additional board until she was weaned, probably because I was already supplying the feed and they were on then on true pasture board. In retrospect maybe I should have. I think 1.5x board sounds fair. Then I would charge full board for two horses post weaning. I would establish with your boarder that “weaning” occurs at 4 or 5 months, or whatever you feel is fair and then whether or not she has weaned the baby is considered a separate horse. Otherwise the boarder may never wean! I’ve seen this happen at another barn, so that’s why I mention it.
She foaled at the vet’s home and I want to say he charged me $350 for the foaling (plus medical supplies/vet fees you would expect to pay for testing, etc.) on top of regular board cost. I forget what board was there, maybe only $300? mare stayed a couple weeks before birth and then a couple after. This was several years ago, grain and hay have gone up significantly (unfortunately).
I agree with those who encouraged you to draw up a specific contract. Just get it all out in the open now.