That's an OPEN ENDED QUESTION ~~~~
Cost would vary dependent on SO MANY VARIANCES. Such as Location, (part of the Country / or City your located), quality of the work, amount of work (prep - which is 95% of the Paint job). Flat is not a difficult Paint Job to perform, it tends to hide a lot of imperfections. You could, if you have a mind to, purchase a Kit that has everything as far as the Spraying Material goes,about 150$ (see link >) Item Detail
and get a Gun from Harbor Freight (low cost), If you don't have a Compressor, you could rent one, and take a day, and Paint it in your Garage or Driveway your self.
You all so have the option of using one of the Chain outfits like Macco or One Day Paint. They tend to be on the less expensive side, from say 300$ on up. Again, depending on the amount of work / prep that needs to be done.
At any rate, should you go with the Flat Black, don't forget to use the LOW LUSTER
Clear Coat over the Base for UV PROTECTION. Otherwise, it won't be too long that that Black will be a Light Gray. There are Paints that have UV protectors in the Paint, but again, they only tend to SLOW the fading a bit. (Florida has some pretty intense Sun and UV's remember)
Good luck should you take this on, and Post Photos if you do, everyone enoys Photos.