When Barca bought Lewandowski he was 34. If he plays well until he is 37 he costs them his wages plus £13m per year for those three years.
If Kane plays well until he is 37, 7 years at £11.5m per year would be £80m. (A bit less per year as he isn;t as good - but not that much less as it would be more of Kane's prime years).
I'm not saying either of them are necessarily good deals - but they are comparable.
According to this website Van Persie's transfer (aged 29) would be £74m in today's money:
https://www.totallymoney.com/content/transfer-index/data/#filters?/options?search=van persie
My view on the Van Persie transfer at the time was that it was a hell of a lot for a player of his age with his injury record. We got one amazing season from him and then two disrupted by injuries and managers. Still has to go down as a good signing overall.