Nasty, I'm not sure if you asked because it is a money issue or if you are just wondering. First off if it is a money issue you may want to rethink having 2 dogs. Getting a dog fixed can be one of the least expensive things you will need to do to properly care for and raise a dog.
I'm afraid to ask you but I'm going to anyhow. Most vets go over these prices with you when you get your dogs their proper vaccinations. Have you ever even taken your dogs into the vet? Given their ages you should have had a whole series of shots done already on both of them. I find it hard to believe you would not have some idea how much fixing your dogs would cost.
Locate your area Rottweiler rescue or another responsible rescue center and let them know you landed two dogs, have your hands full and could use some education on what to do. A lot of breed rescues will give assistance in a spay/ neuter thing because they do not want to see any more puppies on the streets.
Also here is a site that goes over some tips to prepare your self for vet cost:
http://www.hsus.org/ace/11875
We will give you all the help we can but I really think you may be in over your head with this breed and you need to dig in and get busy and do some research.....;)