A player of the same quality as Kane will not be released for under 100 million, but it may cost more. I know Kane scores too high for his age, but that's Tottenham's way of keeping Kane from going anywhere. Tottenham still really need Kane as their main striker next season, but that is not very good for Kane's achievements at the team level.
Kane is a great striker, but he hasn't won anything with Tottenham even though he is a striker who can score dozens of goals every season. So Kane needs to be able to make decisions about his future, meaning he has to force a move in order for his title ambitions to rialize.
If Kane is not traded this summer, he will go free next season. It's probably better to get 100 (90 or even 80-70) million than to lose a player for free. I doubt that a club like Tottenham can afford to "pay" for any player 100 million (plus salary) a year. But that's exactly what will happen if Kane stays for another year and then leaves - it will turn out that Tottenham will lose a player and not get 100 million.
Harry Kane has done a lot for Tottenham over the years. I don't think he owes the team anything, and I don't think the team has any reason not to let him go. This Kane issue is comparable to Mason Mount's at Chelsea. It will be impossible for any team to pay $100 million for Kane, especially when they are aware that his contract is due to expire and he will be available as a free agent next season, so why don't Tottenham accept the price of any team interested? Tottenham should just agree to any price and not take the chance of making money off of Harry Kane, since I'm confident they will get more than $50 million for him.