[Leica] How much to charge for photo in these circumstances

[mvolo at duke.edu (Michael Volow)]

(Leica content- made with Lumix FZ-18 with Leica lens)  I made and 
distributed some photo greeting cards from Paris to friends and colleagues.  
Now one of these folks is interested getting a picture that I used, for his 
business thank you cards.  He would like to purchase the use of the picture 
to have a printer make 100 or so small (~4" x 5")cards.  He will arrange for 
the printing. Do you have any idea what I should charge for such a use. I 
have ever sold photos for this use? What should the agreement specify about 
how long it can be used, etc.?  

The picture is enclosed.

Thanks Mike Volow