A number of years ago, we had our 15 cat feral colony trapped and neutered at one of these clinics. Shortly thereafter, a disease ripped through the colony, killing all but four of them.
You run the risk of someone bringing in an unvaccinated cat, or a cat with some other communicable disease. The vets and techs do their best, but there is a huge chance of cross contamination. The goal is to neuter as many animals they can in that day, the $20 barely covers the cost of supplies.
$350 does seem a bit high to me, at my vet it is a lot less than that. Unless you are really fond of your vet, shop around a bit.