The Northern Echo are reporting the deal to be nearer £11 million:
https://www.thenorthernecho.co.uk/sport/25056019.sunderland-agree-deal-sell-tommy-watson-brighton/
...The deal, which is understood to be worth around £11m, will officially go through when the window reopens on July 1.
....with the 18-year-old set to enter the final year of his current contract this summer, Black Cats chiefs have decided that this is the right time to accept Brighton’s latest offer.
Watson had shown no willingness to sign a new deal on Wearside, and the Sunderland hierarchy did not want to allow him to enter the final 12 months of his contract with his future uncertain.
They believe the £11m deal is a good one for a player who has only made seven senior starts in the whole of his career to date. The Black Cats are also understood to have secured a lucrative sell-on clause as part of the agreement with Brighton.