Dealing mainly with very young kittens, mine are usually too young to be neutered before they go to their new homes, but I do ask people to sign a contract to confirm they will have it done at six months. When I do get older ones in, they are neutered before they are re-homed, paid for by myself with a bit of a discount from my Vet (which may end soon as they have been taken over by CVS). The donation I ask for is £60 so really this only covers the vaccinations, microchipping etc, but I think it is important that any that can be neutered before they are homed are.
Have finally got myself on my microchipping course in May though, so will save some money there at last.