Coming from a composer, (Hmm... let's see if I can answer this without any self-advertising.

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Well, as everyone else has said: "It depends."
Obviously, established composers who've done high profile work are going to be at the upper end of the pay-scale. Also, there are plenty of composers that are willing to give indie devs somewhat of a break to be more budget friendly. (Which is also a very "it depends" thing)
The other main factor of the cost depends on the nature of the actual piece you want produced. Electronic compositions made using sound banks are going to be cheaper than hiring an actual orchestra or band. If you want vocal or lyrical content, it is likely to be more expensive because of the extra work involved as well. You'll have a much harder time getting an exclusive vocal track for under $100; you'd probably want to budget at least $200 for that.
Archeia's "$25-$60" figure is pretty spot on for the RPGM community in general, I'd say, assuming we're talking about instrumentals. As a side-note, that's a pathetically small amount compared to "industry standard" composer rates, but those are based around film, radio, tv and other broadcast media so it's a whole different system. Video game music is kind of an afterthought to the "Music industry" at large.