she asked what I'd like for compensation.
100% of it, just like an injury claim -- no win, no fee!
I've never been in this situation, but I would think it would depend on if it's been published before (some publishing contracts can put a fast halt on your ability to 'sell' your work to someone else for specified period of time. at least that's what I've heard.)
Generally speaking, works already published will have had some form of agreement at the point of acceptance stating a term - you relinquish a degree of ownership when a piece gets published - this is known commonly as first right, and covers the first rights to publicly produce the piece. You can't reproduce it either until the end of the agreed term unless stated otherwise in the agreement. Carefully re-read any agreements to get the full scope.
If you've written a poem specially for the novel, or it is previously unpublished, it's a different matter. You could do it gratis or ask a rate per word -- or, depending on how hard he/she wants the piece, you could bleed him/her dry! Whatever you choose, draw up an agreement for what is paid for the poem and whether you are due extra royalties or a profit percentage per book sold, after all, everyone who reads the novel will also read your poem. This is why commissioned poetry usually has an agreement that purchases the entire piece, including rights and ownerships. Just makes sure everyone is on the same page.
