Apparently its £20m for Sanchez, and £15m for the agent..
http://www.dailymail.co.uk/sport/football/article-5290759/Alexis-Sanchez-says-goodbyes-Arsenal.html
http://www.dailymail.co.uk/sport/football/article-5290759/Alexis-Sanchez-says-goodbyes-Arsenal.html