I think it's a bit of both Stevie (peak years AND being English). For arguments sake, Aspas had a great season at 26/27 on the back of playing in the lower leagues and working his way up, he cost about £7m. Similarly, Lallana has just had a couple of great seasons at Saints and is roughly the same age as Aspas was last season. Big difference in price though. I'm obviously not comparing their respective talents, because I think Adam is a far superior player, but we could have paid premium for Aspas (more than we did anyway), because he was peak age and playing the football of his career.
Just for arguments sake, Aspas could be considered a 1m pound player who we overpaid 7 times his value for, due to one year of good performance.