For most people there's no point in spending any of that money until they've changed the outer cable where it curves round to the rear derailleur. They'll be amazed at the amount of rust and crud that has accumulated at the cable ends and staggered at the drop in performance that they've grown to accept over the months that they've been failing to carry out this simple but important maintenance task.
LIkewise all those riding around with rear mech out of alignment.