How much should I charge to paint a small outdoor deck?

I have a neighbor who has asked me to paint his small outside deck and three sets of outdoor stairs plus some trim work on an enclosed three season porch. He will pay me of course but I'd like to know a ballpark figure to charge him. I don't want to charge too much nor too little as we agreed that it is a job that I am doing for him. Any suggestions? Thanks for your answers. The deck is about 10' by 8' and the three sets of stairs have three steps each. The trim work is minimal.