The numbers started getting silly a looooooong time ago - I don't really understand them most of the time, so I just try and look at them as relative figures as opposed to absolute figures. If I was an Arsenal fan and they'd bought Isco for £70m, I'd start off thinking what the feck have we done but then I'd look at what United have paid for Pogba, I wouldn't feel too aggrieved. It wouldn't be great negotiation, admittedly, but in terms of the standard of player I don't think United would've actually got better value for money.
I do agree that Juve were in a much stronger negotiating position than Madrid which is why they're going to get a significant amount more for him. I just think, with that taken into consideration, £70m isn't that absurd as an asking price. It might well be a reasonable figure for his "actual worth", as you say. I think they'll go for a lower offer too but if they were to get a few clubs in a bidding war they could possibly get something close to that. Which on the one hand is absurd, because he's a starter for neither club nor country, but on the other hand it's entirely possible he'll develop into one of the best players in the world and that's seemingly the going rate for a player of that quality.