I believe the speech evaluation I had done for my son cost around $250. It was not covered by insurance. I did luck out and my insurance covered some of the speech therapy appointments but they were out of network. But something was better than nothing.
My son did not qualify through early start and the private speech evaluation indicated he was borderline. He might have gotten better on his own, he might not have. We could have chosen to wait and see, but I didn't want to do that because I was seeing signs of frustration. My son was closer to 2.5 years old though. He was saying a number of words but about 1/3 or so what his twin brother could say.
He benefited greatly from speech and we only had to take him for about 6 months to get him back on track and going on his own. It was a hassle because the location wasn't convenient, but is was well worth it. I'm also glad I didn't wait any longer.
I don't think my kids spoke much at 20 months. If your son's receptive skills are really good, then it could be that he is just a late talker. It might also explain why he didn't qualify for services through Early Start.
But, if you aren't comfortable with the situation and want to get him checked out and you can afford it, then I say do it. The worst thing that could happen is that you spend the $ and the evaluation says he is fine. That would at least give you piece of mind. And if the evaluation says he is okay, you can still ask the therapist for suggestions that you can do at home to encourage him to talk more.
One thing we worked on at home was always describing what we were doing, no matter how simple it seemed. "I'm going to the refrigerator to get you some milk. Now I'm pouring the milk into your cup. I'm done now. Here is your cup of milk."
Another thing was to interactively describe what he is doing and build on it. Let's say, he can say "ball". When he says ball, you can say "Ball. Blue ball." And if he isn't saying ball, when he is playing with a ball, you can still say it. Repetition helps.
Good luck!