As for prices, they tend to be negitiated as a one-on-one thing, and it would depend on the details of the project. Obviously, more experienced composer/arrangers who are a fair way into their careers will charge more, whereas young whippersnappers - like myself - will do the work for a lesser fee.
If you give me some more details, I'm sure I will be able to offer some assistance.
Ok. If you could give me an mp3, or the score for the song, then I'll have a look and see what I can come up with for you. You might want to do it in a pm to stop someone nicking it though!
Might be an idea - if you can - to get a couple more people having a go. That way, at the end you've got more than one potential direction to go in with the orchestration.
Only one person will have the job - but the others will have gained experience from their pitch, and another piece for their portfolios. Everybody wins!