Pricing is always such a tough area for me. You don't want to under-sell your work, but you also don't want to over-charge and have people think 'really??'
Also not seeing the art and photos themselves, it's hard for me to suggest a specific price for you. But you definitely want to make some money beyond the cost of printing, as well as the time you'll spend trying to sell. Were you thinking of setting a table up somewhere to sell, or just word of mouth?
If you're setting up a table, maybe set out a big sign for what the money will go to (like say it's for Girl Scouts Gold Award, the project she is paying for, and the amount you need to raise, like a thermometer graphic you can mark each time you make a sale). Then keeping it open to donations where the buyer can pick the price? People will probably tend to pay a fair amount if they see it's all going to a good cause! Maybe throw in an extra like a matte for the print if they'll pay $5 more? Maybe also you can put out smaller items with a set price that are cheap (like baked goods or little easy crafts) to draw people in for the lower priced item, which will then cause them look at the art.
Just some suggestions - hope they help! Good luck