Unfortunately I don't know of any vet in NY, but the vet I go to charges $150 for the neuter plus meds.
Breeder that offer their males neutered tend to have a deal with their vets to do it for less, specially if they need a lot of neuters. For example, the breeder I got my pair from, has this kind of deal with the same vet I go to, except that they only pay $75 the neuter. But then again, they neuter all their male joeys that are sold as pets, plus all the males they get from the rescue they run. Not too long ago they took 9 of their joeys to be neutered all at the same time, so even with the discount they spend a pretty penny doing this.
The reason breeders offer a discount price on neutered males when selling them as pet only, at least in the case of my breeders, is to improve the breeding lines, by preventing people from breeding gliders that don't have a lineage. The lower price serves as an incentive for people to buy the neutered gliders instead of the intact males, and not having the funds or interest in neutering them late on, and consequently breeding them; putting gliders that could have some sort of underlying genetic defect back in the market that could later on breed, transmitting their defect in turn.