I used to work in veterinary hospitals, and spaying pregnant animals always cost more. The reason is that the blood flow to the uterus increases during pregnancy, so the surgeon has to be mindful of the extra vascular activity and has to tie off more veins to minimize blood loss.
Therefore, surgery at this time has a higher risk factor- personally, I would let the rabbit have her litter and spay her once she is done nursing them.
FYI- vets also charge more to spay nursing animals because of the increased blood flow to the mammary glands and their engorgement with milk.