£20m would be a mistake. With several clubs competing for him, it's pretty much assured he could walk into their teams, particularly West Ham. We all know how talented he is, the record he has, and that with a full season of playing every week, he could quite easily get 15-20 goals at a club like West Ham.
We could sell him for £20m, or do the smart thing and loan him out for a season before selling him for £35m next summer - or even keeping him if he improves in key areas.
He's got a long contract, there's no need to rush into selling him permanently.