Let’s do some quick math.
From 2012-2013 Annual Plan Questions and Answers
“The total operating budget for the 2012-13 plan calls for $42.1 million in spending”
Let’s state that on a monthly basis, so $42.1 / 12 = $3.51 million per month.
Then, from Wikimedia Report Card:
483 million unique visitors (per month)
Therefore the cost per month is $0.007 per unique user per month. That’s less than a penny a month per user, or 8.4 cents per year. I don’t know of a better value in a charitable donation anywhere.
This means that if you are a good citizen who donated $20 to Wikipedia, the next 230 people you see walking down the street, you can walk up to them with a big smile and say “I paid your Wikipedia bill for the year. Enjoy!”