if this wasn't a reoccurring UTI or incontinence that the vet was trying to get to the bottom of, I can't see why they didn't just send you home with antibiotics and meloxicam(NSAID). It's what rabbit vets usually will do, unless the problem is more complex and they need further testing, which testing the vet themselves should be the ones doing and not sending the owner home to do it, unless the owner requests to do so. So if this was a first time thing with your rabbit, where prescribing the appropriate meds would have likely resolved the issue to begin with, it almost seems to me like the vet was intentionally trying to jack up costs.
I would phone them and tell them the testing kits were useless. That there's no way you could get a clean urine sample from your rabbits on your own. And that you don't understand why they would give you a test kit that couldn't realistically be done by a normal rabbit owner at home, and charge you for it. I would be insistent and demand to be refunded.
For a normal case of a UTI with no complications, additional health issues, or this being an ongoing problem needing further investigation, I wouldn't expect to pay more than $100 for the office visit and meds, $150 at most, per rabbit. But office visits for my vet are only $50, so I don't know if that's the typical cost for you there, it was an emergency visit, or if it's just that vet.
I personally would find a different vet if this vet won't just send home meds for your rabbits and wants to keep charging you for things. Or it's even possible this isn't a UTI but a behavioral problem, or maybe some other health issue. If you want to post what's been going on that made you think it's a UTI, besides just peeing outside the litter box, maybe we can provide some insight or info on whether or not it could be something else instead.
https://rabbit.org/vet-listings/