I know what you mean by the cost
: mine are $90, the kids are $10 and DH's are free (he has a degenerative eye disease).
Basically....you will either have your child on your lap or in the cool chair and the eye doc will look at their eyes with a light. Sometimes this works, sometimes not. Then they will turn off the lights and put the cool pictures up on the wall. Don't worry, they don't put that big lens thing on them, at least mine doesn't. The doctor will then try to get the child to name the things up there, and will repeat this with smaller and smaller pictures.
With a nonverbal child, simple yes or no answers will work (like is there a dog?). They also use a picture book with different sized pictures and have the child point.
Basically at this age unless they have very bad eyesight, it is hard to tell what their eye strength is. Like DS is 20/20 or 20/30. DD just has healthy eyes. They are basically making sure that the children's eyes are healthy, can focus properly and to get them used to seeing an eye doctor.
We go every year (DH gets an exam every 3 months or so), just to keep an eye on things. DH's disease is heriditary so our eye doctor checks the kids for signs of it as well as signs of a lazy eye (I have a very bad one).