I don't know if it's realistic, but if we have a real chance to sign Sancho this summer on 100-120 M we shouldn't think it twice. The guy would come here like a glove in our hands, and he could be a player for a generation. He's got everything and more.
The only thing that could happen if he keeps evolving at Dortmund is that it will cost even more, and maybe some clubs like Real, PSG get involved at some point and make it more difficult for us, having to pay extra wages, etc.
So if we have the chance this summer we have to do it without hesitation, but the main question is if they are open to sell on these numbers or not. Because from Dortmund's point of view it would be better to keep him, so they can get more and better performances on the field for some time, and involve more clubs in a transfer battle in the future.
On the other hand there's the risk of the player stagnating (very low probability in my opinion) or getting a serious injury or something and losing his value. So if they get a big offer and the player pushes a bit for the transfer, I can see us having our chance. They paid around 5 M. after all, and with 120 M. in their pockets and their control of the market they can build a whole team and get some other diamonds. So I think the door is open for everything to happen.