Between my own dogs and rescues, I've gone through ACL surgery with a lot of dogs and with, at least as of a few years ago, all of the surgical procedures used. The fishing line surgery (sorry can't remember the more official term) is the least expensive. The other two are TTA and TPLO (again, sorry don't remember the official names). The orthopedic vets I know are no longer as enthused as they once were about the TTA surgery, and I'd be surprised if that's what you're getting recommendations for. The TPLO is the procedure most ortho vets are going to recommend, and yes, it's expensive.
You don't say where you live. I'm in Colorado, and I had double TPLOs done on one of my dogs 4 years ago for about $6,000. You have to weigh the extra rehab burden and difficulty for the dog in doing both legs at once, but there is a bit of a financial relief in it since things like anesthetic and administration of some drugs are only done once.
Are you sure the vets you talked to who said there was risk of re-injury after the surgery weren't referring to right after the surgery? The TPLO surgery involved actually cutting the bone of the leg and putting it back together with plate and screws. That means the after surgery and rehab periods are crucial, and yes, if you don't take care during that period there's quite a risk. That bone has to heal and heal the way it was put together. Your dog has to accept confinement and potty-only walks. With some dogs that means a period on some kind of drug to keep them calm. However, once the surgery heals and rehab is complete, the dog certainly can go back to normal activity. My double TPLO girl did Carting (pulling less than skinny me in a cart), Drafting (pulling her own weight in a cart over a half-mile cross country course), obedience, and rally after the surgery.
The fishing line surgery is the only one where there would be any possibility of re-injury, and anyone quoting you $4,500 for that surgery is a highway robber, not a vet. It can be and often is done by regular vets as opposed to board certified ortho surgeons, who are the only ones you should consider for TPLO.
The surgery is going to be expensive. That's a given, but if you call enough vets in your area there should be a price variation because if nothing else there's variation in what you're paying for. Some will include drugs and after-care appointments in the price quoted. Some won't. Some add cute extras. One hospital I talked to had "home visits" from some kind of therapist included. Foregoing that kind of nonsense might lower the price.
Anyway, research the heck out of this. Question any vet you're considering using up one side and down the other. Make sure you understand what they're talking about. Make sure they're experienced with the surgery. Cruciate surgery isn't something you have to sign up for tomorrow, take your time and get comfortable with what you're about to do. Find out about credit options if you need them. I'm not sure if vets have to do something in order to offer that Care Credit program, but I know it's available at some vet hospitals. You might see if there's a lab rescue in your area and call them. They probably have experience with the surgery and may recommend certain vets, may have connections to lower cost vets or know of other resources.