It's all been said but I'll throw in my hat and offer my personal experience on the matter.
I think one of the main problems is that a lot of people who give quotes are hobbyist musicians or aren't relying on music for livelihood. So when they start to get paid, they don't need to really get paid all that much if anything. Everyone kind of thinks "well, i've got a friend who does some music, he can probably do it" and so audio's value becomes grossly undervalued. You're paying for the prestige and fans of a composer to come to your game, what can be a huge gulf in creativity and uniqueness between two composers, for their experience and reliability in doing the job, for their time and the masses of work they have to do, and for the wonderful, incredible, beautiful contribution they can make to your project.
Hobbyists may also tell you that a five minute track will take them half an hour to do. Don't let that skew your idea of the workload of an audio guy. I can only approximate but from my experience a 3 minute track may take around 2 or 3 days (minimum) to conceptualise, sequence, record, mix, master. Composers would be capable of making a 3 minute track in half an hour but I would hope it isn't something they'd be proud to release under their name. This estimation is also assuming that you'll get ideas quickly and you won't have to revise the track too much. Be wary of the quality that comes from a quick job.
I completely agree that rates, at least for me, are dependant on so many things - my workload at the moment, how much fun the project looks, how promising the title may be, how long it may take to complete, etc. etc. etc. But, as someone who is on the cusp of turning freelance (well, striving to) who is also charging something similar to the price in question, I will tell you, I work all day and night and for that price I'm not getting the bills paid. Audio freelancers who do an outstanding job need to charge that kind of price at an absolute minimum to not be homeless. It's just a shame that the general conception of price is based on the quotes of hobbyists etc. etc.
The idea of chip music vs. epic orchestral being the two sides of the price spectrum is a little naive I think. Same with composers getting paid per instruments/staves, as you have to define the price of the work by some quantifier and it seems per minute is the most common and in my experience the least problematic.
There's no real objective worth of any track, so guess-work isn't really needed. I suppose it's about people trying to find a good working relationship and being open about what their budget is and what they're after.