At \(2.50\) pounds each, \(100\) items would cost \(250\) pounds. The cheaper items reduce this by \(0.42\), and the dearer items increase it by \(0.58\).

The actual total is \(250-0.42+0.58=250.16\) pounds.

Each replacement lowers the cost by \(2.51-2.49=0.02\) pounds. Removing the \(0.16\)-pound excess therefore takes \(8\) replacements.

This leaves \(50\) items at each price, whose equal one-penny adjustments cancel. Fewer replacements would leave part of the excess.

Answer

The total is \(250.16\) pounds. Replace \(8\) dearer items with cheaper ones.