CCL tears are pretty common sadly. Generally due to the angle of the knee I think (and general make-up of the knee) so a dog is born with susceptibility to them taring. It can happen just walking outside, slipping on ice. My last dog tore it fetching. not doing any fancy moves, she just went out to get her toy and came back on three legs. And sadly, if one knee goes it is highly likely the other will go at some point. So long term ensure the dog is at a slender healthy weight (more towards skinny) and proper muscle tone (but not too much) on both back legs.
It cost me $5000 for one TPLO. all vet costs here are higher though.
if it is a partial tear you can consider conservative management (strict limited exercise and rehab) but with a young dog that can be hard. The goal is for the scar tissue to build and stabilize the knee. Would depend on how "big" the tare is and if the meniscuous (sp?) is intact. I'd recommend doing this with a rehab specialist and a good vet ideally. There are braces you can get (but can still cost a bit to have made).
Did a regular vet make the diagnosis? how? I recommend before doing any surgery to talk ti over with a specialist for proper diagnosis and review of all possible treatments (conservative managment, tightrope, tplo, the other one) https://www.acvs.org/
Price can vary by region. For a lab i'd be wary about tightrope to be honest. TPLO is more invasive but pretty much guaranteed.
Your Stories and Information About Labs with Cranial Cruciate Ligament (CCL) Tears
I seriously considered Tightrope for my 44 pound mix. She was at a weight where it was more likely to work (and the Tightrope specifically is a pretty solid fix). But i opted for TPLO as I wanted to be fully active and competing in sports again.