The honest answer: evidence for custom stifle braces replacing surgery is thin, and no brace stabilises a dog's knee the way surgery does. Braces are marketed hardest at exactly the owners least able to afford surgery, which deserves some healthy suspicion.
Where braces have a legitimate place: dogs that cannot have an anaesthetic, owners for whom surgery is truly impossible, and as one part of a properly structured conservative management plan in smaller dogs. In those cases a well fitted brace may improve comfort during the months a knee takes to stabilise with scar tissue. What it will not do is restore an athletic knee or prevent the arthritis an unstable joint generates.
If the brace budget is a few hundred dollars and the surgery budget is out of reach, put the question to a surgeon first: for some dogs, that money is better spent on proper conservative management, weight loss support and pain relief than on a device.
Send what your vet said and quoted. A veterinary surgeon reads it and comes back to you with what they can do and what it would cost, before you pay anything. A written second opinion is from $250.
Send the case, hear back from a surgeon with what they can do and what it costs, and only pay if you go ahead.