from About.com Music Careers:

Who pays for the promotional costs of a show really depends on the kind of deal promoters and bands make between themselves…

• If there is to be a flat fee for the show, that the band gets no matter how many tickets are sold and whether the promoter makes a fortune or loses their shirt on the show, then the promoter shoulders the responsibility for the promotional costs.
• If there is a door split deal, the promoter still shells out for the promotional costs up front, but the band does not get paid until the promoter makes that initial investment in the show back.

As you move up the ladder to large gigs in bigger venues, with bigger promoters, then the deals will be more cut and dry, and you will encounter more flat fee shows and fewer door split deals. In the indie music world, especially in smaller venues and with promoters who work with up and coming acts, the deals are much more fluid. Read More.