$600 is absurd! Granted, the cost of living is much cheaper down in Texas, but I paid significantly less than that for TWO spays.
The best way to find an affordable option is to start calling rabbit rescues/animal shelters (that take rabbits, of course) in your area and asking if they can recommend a reasonably-priced vet for a rabbit spay. Shelters spay/neuter all their animals prior to adoption, so if they take in a fair number of bunnies, the vet they go through will obviously be experienced with bunns. While you obviously won't be able to get a spay done as cheaply as the shelter itself will, whatever vet they go through is fairly likely to have more reasonable normal rates. A shelter or rescue may even be able to tell you about a low cost spay/neuter clinic in your area that takes rabbits.
If you're planning to adopt an already fixed rabbit from a shelter or rescue to bond her with, you could even talk to a shelter and explain that you'd like to adopt a companion for your current rabbit from them but need to find a reasonably-priced, rabbit-savvy vet to spay her first (I would mention that you've been given a price quote of over $600 so they know it's unreasonable prices (rather than financial hardship) prompting you to search for a cheaper option - that way they won't make the incorrect assumption that you may not be able to afford taking on a second bunny and will be most likely to want to help you get in a position to adopt one of theirs). It's a bit of a long shot, I suspect, but you might even be able to schmooze a shelter into finding a way to get your rabbit spayed *very* cheaply (as in maybe $100ish range) if you were also willing to put down the adoption fee for a second bunny at the same time or something like that - they want good forever homes for the animals in their care, so being able to pose your question in a way that makes it in the shelter's best interests to help you get an affordable spay could make a big difference! It's worth a shot, at least. [You may want to try to track down the direct # for an employee who oversees the adoption aspect of the shelter, as the people who field most "default" incoming calls will be volunteers who aren't in a position to strike a deal with you - you'll probably have better luck speaking with one of the few true/full-time employees who oversee things. The actual employees usually have a title such as "_________ coordinator".]