I would suggest that you use the links in several posts here to research the vets in your area. You can find several by zip code, and call to ask their prices.
The problem is that a standard spay is going to cost the price you quoted, especially a spay for a female because it is a more delicate procedure. The only cheaper option might be checking your local rescue shelters and ask if they have a vet they can recommend that may charge less.
If money is a problem now, are you preparing financially to care for these bunnies for the next decade? They will need periodic vet care, along with the expense of daily supplies. You may want to set up a little savings account and set aside a few dollars a week for emergencies. Also, while they are both healthy, consider getting insurance (I use VPI, which is $12 a month per bunny) for them. Last year while being unemployed I needed a sudden vet visit, and boy, it saved me hundreds.