I've never heard of a price difference with age (either from my vet or on any bunny forum). That's a new one!
Spaying at four months is the youngest I've heard. I've always heard that 6 months is the ideal age from my vet and second hand through other peoples vets. There are plenty of members here that spayed their bunnies under 6 months and there's no way to really know how or if it affected them. In the end I think you really have to go with what you think is best.
From MediRabbit:
A spay is usually done around the age of 5 months, or when the rabbit is sexually mature. At a younger age, the surgery is complicated by the difficulty to locate the very thin uterus and the very small ovaries. A spay at immature age may also have a dramatic effect on the proper calcium absorption by the bones.
Warning: Graphic pictures at website- http://www.medirabbit.com/EN/Uro_gen_diseases/castration/Spay/Steril_en.html