I think in principle you should realise full value for a top 3 world striker, especially from a financially ffp ignoring doped club. Don't forget, you could be waving goodbye to 3 trophies because you given the (already) best team a 50 goals a season striker.
Of course our problem is, we were looking for 1 forward with Kane, without him we are looking for 2.
If the fee is known, we are going to be rinsed out by clubs we want to buy from anyway. The only situation this might not happen is deadline day, where desperate clubs are forced into making desperate decisions. Whether we (the fans) can stomach waiting that long (especially if we just have Son to choose from) and a deadline is still a risk...you're fudged til January if you mess up.
Overall, Id rather he stay.