The process of commissioning a composer to write a new, original piece of music can (and should) be an engaging, rewarding experience for everyone involved. I've written works for many situations and ensembles, and this process has been different every time.
That being said, I use the most recent version of New Music USA's Commissioning Guide as a starting point for this conversation. It contains appropriate legal background concerning this process, and includes a typical cost breakdown for varying types of commission fees per minute, based on ensemble size, how the music will be used in performance, copyright issues, costs for typesetting and printing music per measure (which are separate from the commissioning fee), etc.
Of course, most points are negotiable, including the final commissioning fee, plans for recording the work, performances beyond the premiere of the piece, etc. I can provide a final commissioning contract, but I'm also happy to sign a similar document from the commissioning party that properly outlines our agreement.