How much would they have slugged you if it was the other way around ie you allowed for an H1 slab and they said you needed an M class slab. That should be the reduction. I've heard it's around $15k.
From M to H1 slab upgrade will normally be about the same credit that was given..around $3K. This is same estimate we were given with a few builders we asked. $15K is way way way too much.