If they start to offer £200k per week wages they'll have to renegotiate several contracts. It isn't just the £10m it would add onto their salary bill for Sanchez, it's the other £10-20m that it'd cost to uplift other contracts whose agents would demand parity or at least an uplift. Then it's the players who are on a band below who want to be dragged up likewise.
Spurs' fans particularly on here gloat about us having somewhat mediocre players on contracts similar to their best players. However that's what happens when you pay truly top wages, rightly or wrongly other players' salary gets dragged up. If your best players go from a £160k weekly salary to a £260k weekly salary, the squad players tend to get dragged up by a similar % (from say £60k to £100k).
That's why we end up with Young, Fellaini, Schneidelrin, Shaw, Depay etc on £100k weekly salaries.