When you hire a musician, you should agree on how much you will pay them and what the terms of the exchange are before they begin working.
Non-exclusive is the cheapest kind but it takes away some of the benefits of having a composer in the first place since anyone can use the tracks they make.
Limited Exclusivity - for a period of time, is your best bet, but will cost you more since, like a previous poster said, they can't make money from their work elsewhere for an extended period, therefore you would have to pay them more to make up for it. This is often the path I take when people have a slightly bigger budget. I charge them about 40% more for 2 years exclusivity.
Exclusive into perpetuity licenses would usually be more interesting as "buy outs" in which case you'd now own the music made. This comes at an even higher price, but remember, the composer must be credited even if you buy all the rights from them, unless you agree in writing to do otherwise. A buyout would cost you anything from 500 to a few thousand dollars for a single song depending on many things, but mainly who the composer is and what they're willing to do (sell their soul?).
I hope your composer is well informed and that you don't take advantage of them if they are not. Pay them fairly and the happier they are, the better the work they'll do for your film.