If he leaves Spurs most teams in Europe will be keeping an eye on proceedings and thinking about a bid. He will be spoilt for choice when it comes to where he wants to ply his trade. I just think it would be too costly to bring him in with all the other surgery that needs to be done to the squad. When fit he can provide 20+ goals a season, that in itself is worth a small fortune, let alone being English, Levy levy, and his all round play. North of 100mil unfortunately.
But understandable why he will command that sort of sum, amazing finisher with both feet, you do not feel safe as a defender showing him inside or down the line. Good in the air, great hold up play and eye for a pass. Strong on the ball, you cannot man handle him off the ball, well not without drawing a foul. Presses hard if the tactics demand it, model professional off the pitch, team player, happy to drop off and turn provider. The dood is the fastest player ever to 20 CL goals in a good but not great Spurs side. So you cannot even point to him being poor in European competition.
As to the comparison with Martial, last season was Martial's best and arguably Kane's worst. Kane still managed 18 goals.