Most run 10$ an inch....some a fair bit more than that...look around at their fish first! I've seen a lot of fish come out lookin like a clown....I wouldn't want some ridiculously painted fish on my wall....good fish painters are few and far between...and the guys that can do them typically charge a bit more....
That price usually doesn't include the drift wood or plaque it will be mounted on. More elaborate forms; curled left/right, mouth open, open gills can run you more also.
I would shop around and look at work not price. At first you may get sticker shock, but you will enjoy the mount for years. In my opinion, I would rather deal with a high price and enjoy it, rather than a cheap price and hate it every time I looked at it.
Painting can be difficult, and I think with rainbows painters tend to go heavy on the red (pink) stripe down the lateral line.