That's interesting. Do you have an idea at what price offering a sponsored free search engine would become profitable? $15 per year seems very low and I would be happy to pay considerably more than that.
Facebook makes about $200/yr per USA/Canadian user. So probably start there.
A relevant factor is market segmentation - the average ad revenue is a bad target for such a price, since the people who are willing to pay $x/month for an ad-free version are the ones who are much more valuable to advertisers than the average.