Usually this is what happens.

The promoter starts to put a fight together by finding out what each boxer needs to get to make the fight happen. Next he'll go to TV (or PPV) and sell them the fight to find out what they are willing to pay for the rights to broadcast the fight. Once that is all agreed to next he'll find a site and ask what they are willing to pay to put the fight on in there arena. This will also include the gate as well (the amount of money coming in from people seeing the fight live). Most fights made in Vegas include a flat site fee and no % of the gate. That way the casinos can comp as many free tickets to their high rollers as they wish (Most big fights in Vegas see as much as a 30% in gambling venue during "big fights"). In Vegas for big fights there is also closed circuit profits to consider as well. If either of the fighter has a large following overseas they can also negotiate overseas broadcast rights (The Haye-Klitschko fight is a good example).

Now if the card is on PPV there is usually a median buy rate to cover all costs. After that fighter will usually have negotiated a share of the profits over a certain buy rate. So if a PPV exceeds expectations the fights can usually do rather well. The promoter is responsible for stocking the undercard and paying those fights as well. TV is usually responsible for advertising the fight.