I am sorry to hear about your son. My son is almost 21 months old and only says "go go" for go. He has never said mama, etc. I have some input from my own experiences in this area with my son.
As for the cost of speech therapy, I believe Easter Seals charges $66 per 30 minute session for speech therapy (they may also decrease that rateif insurance doesn't cover as I was told a discounted rate for other evaluations that my insurance did not cover). We had our son evaluated by Jennifer at Easter Seals and I was very impressed with her and the speech therapy program. My son is currently attending speech therapy twice a week at Baylor-Our Children's House due to the close proximity to our house.
Also, as for at home activities, we are doing the following things with our son (as was recommended by his speech therapist):
1. When talking with your son, tell him "Mommy says boat (or whatever word you want him to say)." then say "(your son's name) says boat" and wait to see if he will make a "b" sound or try to say the word. It is supposed to be easier for kids this age to hear their name and know you want them to say something then to say now you say... because children may not know pronouns yet.
2. Put objects up to your own mouth and pronounce the name of it in a slow exaggerated manner with your child watching your mouth. Then repeat the word over again. Such as, our therapist told me to get farm animals and hold one at a time up to my mouth and say, "T (your son's name) wants elephant", then try to get him to say it or some sound for it. When it comes time for another animal, hold it up and say, "T wants more" emphasizing and holding the m on the word more.
3. If your child becomes frustrated, don't push the issue, but do get him to calm down and not associate screaming/crying as a means to get what he wants. So when my son starts screaming because I am not giving him what he wants, I calming explain he needs to calm down and then I will get it for him. When he calms down I hand him the object.
4. Our Speech therapist recommended our son learn basic baby sign language in the interim to help reduce his frustration and aid in language development. Luckily for us, we already had started this learning technique. There are baby sign language videos out there, such as the "Signing Smart Series".
5. We also have been told to put together a photo album or other type of picture book of common objects, label the objects and ask our son to point to a certain object and when he does for us to name it slowly and ask him to repeat it as discussed above.
6. Expose your son to other children around his age that are talking. We had an in home nanny for our son until recently and he was not exposed to other children daily. Since we have started taking him to places where children frequent (church nursery, small group child care, play groups, etc.) he has started taking notice of these children's speech skills and at least hears and observes their behavior. My son is now attempting to imitate these children's speech.
I hope my input helps you.