I know it's my clinic. Look at the contract. If you cancel early, at least on a dedicated cycle), you aren't out much money (I think they pay something like $500 to the donor if you cancel before retrieval), because neither they nor the donor has done much. I would ask to talk to the doctor ASAP and tell him to be frank with you - that she's stimming poorly and this cycle isn't going to work out the way it's going. I felt the same way when I cancelled the cycle I cancelled. At first the clinic didn't want to cancel, because they think they can make just about anything work, but they saw it my way after I talked to them.
How old is your donor? Don't scrap it all just because you got one non-performing donor. It could happen to anyone, and you caught it early on in the process.