How Much Is Music,Generally?

Varies massively. Some people put out royalty free music for video games, an aspiring bedroom composer might just want a tenner or so for a simple song if they''re putting music out on Bandcamp/ Spotify anyway, whereas a known pro would want thousands.

Generally, for commissions, I try and find music on Newgrounds similar to what I'm after, look at the composer's page and back catalogue and estimate whether they're roughly on our budget level and if we're likely to work together well before negotiating a price. Factors that can affect the price include:

is the composer well-known? established? in demand already? Is the music hard to find elsewhere (e.g. a jazz sax tune will cost more than an electro dubstep song put together with Fruity Loops or whatever), is the project free or expected to make money? does the composer sympathise with the project? is the game developer established? (the composer would generally expect more from Nintendo than from Mum's Basement Game Studios) Is the project simple? Is the composer being asked to do something beyond what they normally do? Is studio time needed? Would multiple people be needed? Is the composer mainly in it for the money? etc. etc.