If Kane continues to improve at the rate he's shown over the last 4 months AND he keeps scoring in the CL (becoming one of the players of the tournament) AND he does well to great at the World Cup, £200m won't be enough.
If the above happens, his age, in particular, and the potential for more growth, is going to make him the most coveted player on the planet alongside, or above, Neymar.
The sheer amount of goals he is scoring is one thing, but looking like a world class player without scoring is something else entirely. He's legitimately a forward a team can expect to carry them and enable others. Players like that are gold dust and rightly go for more than specialists with a niche do.