I couldn't see Kane leaving Spurs unless it was to a top European club, so this will only add further security of that alongside pushing up his value.
If they really wanted him, then as you mention they'll need to be able to afford his large wages and contend with his extended contract.
Personally, I think you'd be talking about a deal worth north of £100-120m if someone wanted to prise him away, which would be a nice return.
@Yid4life you may disagree, however with Levy being an astute businessman I could see this being a part of an eventual move for Harry.