No, you'd pay $100, and get it displayed for a day, on, say, the front page of Wikipedia. If you're the only person who bought an ad for that day, you show up for every visitor. If more than one person bought an ad for that day, the ads are rotated. I suppose this could be done for each language.
Perhaps ads could be sold for a day on a specified Wikipedia entry, which would expand the possibility of sales, but also increases the possibility for controversy. (Imagine someone buying a birthday message for Lee Harvey Oswald to be posted on the John F. Kennedy article, or a message honoring Himmler on the Judaism page.)
No guarantee of number of page views or impressions. This wouldn't be a money-maximizing advertising scheme, so regular ad-marketing measures would be irrelevant. Avoiding such things would also keep it informal, which might cut down on "OMG Wikipedia sold out and went corporate!" angst.
It'd just be an additional source of revenue that would avoid entanglement and influence issues. If it was popular, Wikipedia could tune it to raise revenue.
