I see where you're coming from with your valuations, but it doesn't really work like that, he would be worth more to City and Barca that 10 and 5 mil purely for the value increase, if he wasnt valuable to anyone else bar us then he would go for a lot less tha 60m.
Doesn't work like that. If City is well stocked on striker area, buying him for 60M and not playing him regularly will only reduces his value. How can his value increase if he doesn't have the games to improve? Not to mention he'll command a high wage, probably not 500k high, but he certainly won't come cheap.
Hence I think city won't even look at him, 5M is just a small number to show that in extreme condition only they'll probably take a punt.
Plus, buying a 19 yo players for 70M is a stupid business IMO, especially one unproven as Haaland.
As for us, what's the purpose of buying a 19 years old? So many cons:
1. He might / might not make it, at 19 you could still go both ways, not every 19 yo is ronaldo
2. Even if he made it, he'll make a lot of error along the ways. For 80M why are we the one who should provide the learning, we should expect ready made player for 80M
3. If he made it, great, but we cant claim he's ours when we bought him for 80M
4. He'll serve us for years? Probably, but it won't come cheap, by the time he's 25 (if he actually made it) he'll command like absurd wages.
5. All the risk for nothing guaranteed
6. We're better off buying proper established 25 years old strikers, less risky, sell him when he's 28. Rinse and repeat.