We sold 7 backups for nearly £100m in 2017.
Chambers and Iwobi are young and have potential, Xhaka is a solid starter for most premier league clubs (excepting the top 6).
Perez is 30 in a few weeks and hasn't done much in the last few years. £4m seems a reasonable price. Moreover, he's more interested in playing than earning which is probably not the same case for Jenkinson and Campbell who are collecting a paycheck and are comfortable. I think Jenkinson was sold at one point with an opportunity to play but he wouldn't accept the wage drop.