Males are usually much cheaper to neuter than females (males can get by
with local anesthesia while females must go all the way under).

Much of the cost will be determined by where you live, how socially
conscious the vets are there and how well-funded is your local shelter.

Oftentimes the shelter procedures are subsidized either through
government grants or private donations of money and/or veterinarian
skill and thus are provided at cost.

Private vets must make a profit to stay in business, so expect them to
cost more. The upside about building a relationship with a private vet
is that you will then have a doctor who knows your pet (and hopefully
comes to care about him/her as much as you do).

Here where I live, the Humane Society receives funds, both private and
public, to neuter trapped ferals and strays. For $10, they will test,
vaccinate, and alter the trapped animal whether male or female. A
downside for many feral colony caretakers is that if the animal tests
positive for disease, they will be immediately euthanized without
consulting the trapper. Quite a few caretakers find this unacceptable.

This low cost program is not available for privately-owned pets,
unfortunately. For pets, the cost at the local shelter is much the same
as what you pay.

My rescue group came up with a list of services and costs a couple of years
ago. it is at http://www.animalrescuefoundation.com/free_kitten.htm

Back then boys were $174 and girls $224. We adopt out for $75.