I believe £10-15 is nothing for a quality player like him these days.... just think about it.
*He is ajax captain at 24, so he is very mature whilst being still only 24 and is a leader
*He is from Ajax academy, therefore from a Cruyff / barca school of thought
*He is tough, tall, and physical (thats something we are missing in defence)
*He is left footed and can be played as CB and LB in 3 man defence
*He reads the game very well and is very smart, therefore will be adapting to the spanish game and our way of football
*He is the long term replacement for Puyol and Abidal
*He is very very good with his feet and builds the play from the back
*He is cheap considering his age, his qualities and the market in general