Mason made a club record £13m move from Tottenham.
He said, “I've not thought about the transfer fee too much.
“It comes down to what you do on the pitch. If I put in some good performances and have a good season, the money side gets forgotten about.
“The main pressure I feel is the pressure I put on myself to do well. I've been at a big club, played in some big games and I've come here to improve the squad and to help the team."