If you find a spay and neuter clinic where that's all they do it can very well be much lower. The place I used was $75 for dogs 40-70 pounds and they do FANTASTIC work, very nice, but they do spays and neuters all day everyday, they're mobile, but that's what their purpose is so costs are lower. I know it can be done at a very young age as our animal control will do it on puppies that are 2 months of age, but if you're not dealing with the shelter and it's just your own personal pup you got from a breeder then vets usually recommend 4-6 months of age. My vet had said 4 months, I waited until 6 and then had it done at the spay and neuter clinic. My boy had an undescended testicle so his neuter was $100 which was a far from from the around $300 my vet was going to charge and they did a beautiful job. I would investigate the $50 place a little bit more, but if they are cheap because that's all they do there's nothing wrong with that, just make sure to check them out, ask questions, meet the staff, etc before making that decision.
Best of luck!