According to this article in Computerworld 10Million$. Yes 10 fucking million.

During a SEO conference in London, Rand Fishkin, president of a SEO firm recieved an offer from a poker site owner. The offer was simple, get a search ranking in the top 5 using white hat methods for an online poker or gambling site and we will buy that site from you for $10 million. Needless to say Fishkin was blown away. He spoke to other gambling site owners and the response was even more surprising. They said if a site can really rank there we will buy it for more than $10 million.

To understand why site owners are willing to shell out so much is because a single online gambling customer brings in atleast $1000. Its easy to see why site owners would pay so much. It seems that being at the top of Google is the most important part of the marketing plan of such sites. Imagine you are running a e-commerce site and you had the option of paying a certain amount to appear on the top of Google results, how much would you pay?