Reiterating what others have said, essentially you should pay as much as you can afford. The composer wants to make as much money as possible. You want to spend as little money as possible. There's a number inbetween that will make you both happy, and Michael's suggestion of a percentage of the budget is a good one (and usually how music budgets are calculated on big projects). Lots of composers will work for free if they're starting off, or particularly love your project, or looking to fill a gap in their portfolio, etc, etc. If you really have nothing to spend, dig around, you can find someone, but try to offer some money.
Advice I could give? If you DO get a composer for free, take them out to dinner afterwards, or buy them a case/bottle of their beverage of choice. Goodwill is worth a LOT and they just did a TON of work for you. If they're doing it for free, odds are they're working a dayjob too, which means a couple months of 15 hour+ days. It's exhausting, physically and emotionally draining (unless they're absolutely phoning it in, in which case you probably wouldn't want to work with them again anyway). Show them you understand and appreciate it, and they'll definitely want to work with you again.