$700 is absurd. My vet was a little on the high side and I still only paid $465 or so for two spays. You'd be better off driving a couple hours away than paying that much!
Here's a thread that lists low-cost spay/neuter options for rabbits. Failing that, you can check our
database of rabbit-savvy vets for one that's a little further away.
Also, the best place to get the second bunny is through a rabbit-savvy shelter (or rescue) that allows bunny dates with their adoptable bunnies (and will also almost certainly allow you to return/exchange the new rabbit if the bond just doesn't work) - you could find a shelter to go through for the new bunny, then explain that you need to get your current bunny spayed before you can adopt a companion for her and ask them if they can recommend any good, reasonably priced options. Heck, there's a chance you could get the spay done through the shelter at an affordable price if you're also adopting a second bunny from them since.
I would specifically mention to the shelter that your vet wants to charge $700 for a spay, as that amount is so unreasonable that it's virtually guaranteed to garner sympathy... not to mention, it indicates that you're asking for help finding a low-cost alternative because your vet is on crack, NOT because you're having trouble affording it (something a shelter could find concerning when you're also looking to take on a second exotic animal).
At the very least, it never hurts to ask! (And seriously, don't pay $700 - that's so outrageous it makes me wonder if the vet is trying to deter people from getting their bunnies spayed there because they're not entirely comfortable with doing the procedure... and even if that isn't the case, I doubt they find many people paying that price which means the vet won't have as much recent experience as someone more affordable.)