This move will have more elements to it than just money. Very few people move for just money in any walk of life - its a fallacy to suggest otherwise. There will be other considerations, playing opportunities, coaching/development standards, location, family - not sure what his personal arrangements are, club culture, external issues - travelling, spare time friends etc.
What is a significant upgrade - if you are on £20k and someone offers you £30k that is a 50% uplift - the fact your peer group might be on double that still makes it a big upgrade. Its the starting point that matters.