Chelsea always seem to get inflated fees for their players, possibly partly due to the fact that their players are all signed to ridiculously long contracts (Petrovic signed a 7 year deal when he joined Chelsea in 2023).
We, on the other hand, are negotiating from a position of weakness, due to us being relegated and constantly flirting with PSR difficulties. Kelleher had one-year left on his Liverpool deal and went to Brentford for £17m, so I think Mads, with 3-years left, will go for a fee somewhere in the middle of Kelleher and Petrovic. About £20-21m.