Looking on Charity Water's site, backing out the math, your per-week cost is a little less than $0.02? If you're looking to mimic to some small extent the success of FreeRice.com you're going to want to make the action trivial - looking at that puzzle makes me want to go away, where clicking a definition on FreeRice is easy, right or wrong. If you want clicks / click traffic / impressions ($), make it easy to click.