About people posting "if he'll play 100 or 150 matches, that means that he will be a success".
Just some numbers, for comparison:
Fabregas, 151 matches in 3 seasons
Alexis, 141 matches in 3 seasons
Mathieu, already 91 matches for Barca
Arda, 55 matches
Gomes, 47 matches
Denis the ghost, 36 matches
I am not saying whether he will be good or bad.
But, at first reports said that we paid him 30M, which is cheap.
And now it seems, if will be a starting RB for even only 2-3 seasons, he will easily come to 100-150 matches in all competitions, which means that we will pay him 40-45M in the end.
30M for a RB who played one good season is okish.
45M for the same player is a lot.
On the other hand, if 45M for Semedo is considered cheap, then Arsenal probably asked 60-70M with variables for Bellerin.