First off, there's a good bit of work involved with preparing a car for paint. This is by far the most important step in the process.
You'll need to sand the paint to blend in all surface discontinuities after first removing all dents and dings from the body surfaces.
You'll need an air compressor and spray gun as well as lots of sand paper, thinner, solvent, primer and reducer.
If you have all that, you'll want to have the advice of someone who has actually painted a car to help you through some of the problems that will arise.
I've done a couple of paint jobs and they turned out well, but I had the help of a friend who owned a body shop to talk me through the process.
Your cost to do this depends on what tools you need to buy and the help of someone experienced with this process.