Prices depends totally on the composer. Some composers are asking per-minute prices, per-track prices, or per-number of instruments in the track prices ^^'
Per-minute and per-track prices tend to work out to about the same since they are based on the same scale. A general rule of thumb that my comp teacher gives to his students is that a decent composer can work at a pace of about 1 minute of music every 3 hours (for most "chamber music" type things)... but that's before you have to do any sequencing or rehearsing.
Sequencing is its own animal. I know composers who are good at composing and terrible at sequencing, and I know guys who can't compose at all who can do insane things with a computer... and a lot of people spanning the gap in between.
The only time you might need to worry about the number of instruments in a track is if there are live performers involved or you are asking for an orchestration that is outside of your composer's comfort zone (asking someone who usually just does electronica pads and piano to write something for a full orchestra). In the latter case, you might end up better off trying a different musical vision or a different composer (depending on your priorities). If you really have money to spend, you can hire a composer and an orchestrator.
The best route to go if you are worried about budgeting is to try to work out some type of package deal from a composer. There are a fair amount of decent composers running around who'll at least consider an offer if you go to them and say, "I'm putting together a VN, I need 15 tracks: 10 tracks @ 1-2min, 3 tracks @ 4min and 2 tracks at 5+min (for opening and closing sequences). I can pay you $x for the whole thing (half up front and the rest when we're done)."
Heck, I did a 4min drum track last fall for some friends to practice belly dancing with for a case of beer (which I drank while making the track) and some homemade cookies.