You make some good points but as you say there is potential there and aspects that require improvement and a club that is confident in their system and coaching may be confident they can develop a player further. (I’m a coach so I’m always gonna big us up!)
So as has been mentioned sometimes you pay for the potential and take a calculated risk.
As to the exact amount he’s worth I have no idea, all I know is with a bit of luck we still get a great player performing for us or we get a club that overpays so it’s a win win