In my humble opinion, even if Paterson doesn't improve he will be easily worth 20M to Everton as they are a Premiership team. The fee's Premiership teams pay are inflated, by most other league standards. Can I ask you, what is Teirney, Robertson or McGinn worth in World terms today? Care to take a guess?
Nathan Paterson will be the Scottish International right back for a Decade, for what it's worth. Premiership teams pay a fraction of what they would pay Championship Teams for similar players from any other leagues.
Paterson's current ability is more than appropriate to the fee Everton has paid. I'm old enough to remember when Dalgleish went to Liverpool... the question asked then was. Could he replace Keagan, was he fit? History tells us all how that turned out, now didn't it?