From a purely financial perspective, I disagree that he is worth more to spurs than City.
For City, he could be the difference between winning the champions league or not. That would bring in lots of extra sponsors and additional prize money, which for the champions league is a large sum.
For spurs, next season kane staying or going won't make too much difference. Realistically you aren't going to be challenging for the title and or even getting top 4 so kane staying might make the difference between getting in the europa league or the europa conference league if you have a decent season or the europa conference league and nothing if nuno gets off to a slower start. Either way, financially there wouldn't be a great deal of difference.