In Cardiff, where I'm from, the local PDSA (People's Dispensary For Sick Animals) which is a charity offers dogs and cats in conjuction with Dogs Trust (Rescue and Rehome Charity) and Cat's Protection (UK's biggest feline rescue and rehome charity). They are priced around £20 for a female and £15 for a male cat. £25 for a female dog, and £20 for a male dog.
So I guess that would be around $40 tops for the cats and $50 for the doggies.
I think in Britain female cats are spayed differently? The surgery is not evasive at all, they are done from the side of their body with minimum scarring and the fur just grows back. I think somebody before on here has pointed out that it's a different procedure in the US to the United Kingdom.
The PDSA offers vet practice fo people on low incomes who recieve housing benefit. But you can use the neutering, worming and flea services even if you don't recieve benefit.

Jack & Sunny were already neutered when they went into Cat's Protection by their previous owners.