How much for a paint job on a 1989 mustang?
Someone else just asked a similar question about a Camaro: It really doesn't matter what make it is.
The condition of the body is important and what you want from it is important too: Just to drive? Want to show it? Body is rough and needs work?
The price of paint can vary from $299 [paint over the dirt] to $5k for a really good quality job with wet sanding and multiple clear coats [which makes the color last, the finish really look good, and provides a longer life].
My suggestion is to go to a local car show and ask around to get some recommendations for local body shops.