I think the idea is that he'll be attacking more than he will be defending so we need a fullback that's great going forward. The price is irrelevant it's what we were prepared to pay and spurs were prepared to sell at end of story. Walker right and mendy left (hopefully) is very very exciting.
It seems to me that Guardiola has determined that we need athletes in the full back positions to implement his style, players that can press high but have the capacity to recover their position should the need arise. I may be wrong but with Walker and Mendy not the most gifted technically that's how it seems.