Having said that here are some ideas for working with charities where you are not giving the both away for free:
a)CHARGE FOR THE PHOTOS- and then split the price between you and the charity.
b)SELL AD SPACE ON THE PHOTOBOOTH PHOTOS- limit the photos to 2 or 3 so that you have generous space left over for the advertising of the sponsors logos- once again make sure some of the money goes to you and some back to the charity. Remind the sponsors that unlike brochures or postcards, portable photobooth pictures rarely get tossed after the event. They end up on bulletin boards, fridges and many times on social networking sites such as Facebook. Done right, photobooth photos can be a valuable advertising product with shelf life.
I generally will give a reasonable discount to registered charities, more so if it is on an off day, but I won’t go overboard. The person who gives their work away may get a lot of work, but at the same time they’ll go broke. Then the result is that you are a non-profit or charity yourself. Pick a few to work with that have a special meaning to you and give the others deals on non-prime days.
Also, if word gets out that you work extremely cheap, well you will devalue your product and will have a much harder problem getting a reasonable price for your services.