First, you to find out the going sq/ft pricing in your area. This can be done by talking to your local paint rep, they are constantly talking to builders.
After you get this info you now need to know the specs for the job. Example- one color and sheen or other combo, is trim and doors to be included, interior only or exterior and interior? Getting this info will determine the sq/ft cost with the cheapest being blow-n-go one color and sheen. Now do your measurements and go to your local paint store, the same one with the rep you talked to, and negotiate a primer and paint costs.
It has been a few years since I bid this way but all of these ideas are still important.