Composers for games charge anywhere from $200 to $2,500, per minute of finished music [1]. The former number would be a composer just starting off their career (assuming they have some base level of skill, from a degree or prior practice), and the latter would be an industry veteran.
During the final stages of producing the soundtrack, master all the tracks (much like mixing, except the goal is to make the frequency distribution and apparent loudness seem consistent across the whole soundtrack).
A full buyout is the norm for AAA games, where the composer doesn't expect to see any additional revenue after what he/she has been paid up front for the work. In indie titles where funding is more tight, it is much more common to trade off some of this upfront payment for things like game revenue share and soundtrack sales.
If you are a relatively un-established indie developer (which I'm guessing you are from the nature of the question), you would probably aim for a similarly experienced composer who should be charging anywhere from $200 to $500 per minute of music. If each of your 20 pieces needs to be a minute long, that is 20 minutes of music, or anywhere from $4,000 to $10,000.
Keep in mind that this is for a full buyout of rights, meaning you as the developer own the music fully and permanently once it is created. You are free to use it for any purpose you want, including releasing the soundtrack for sale, using it in future game titles, using it other media such as television, etc.
If you think this is just composers being diva's, think again. I think you'd find that most composers don't like dealing with money, and just want to get to the composing. But we have to do it. All other fields in game development - concept artists, pixel artists, animators, modelers, programmers, etc. - charge for their time too.