Laugh while telling them $100 or best offer.
Actually pricing for a single poem would be hard, depending on how it would be used. If they are just copying it for themselves, I would just give it away. (Just me there) If they were adding it to a book to be published... Add my name in there. (Again just me) If they had grand plans of doing a poster set up I would ask for the first copy.... just to make sure it looks good mind you.
But as I have given away more copies of my poetry books than I will ever sell... (Mainly cause I have never tried...yet) I would just just let them use it.
Unless of course they are willing to buy me a house ooff the sales of it.