I return to data from Fantasy Premier League, used previously, to demonstrate player productivity and opportunity cost. It should be noted, that player values are not fixed, and do rise and fall as performance change. The pictures below present the most productive Fantasy Premier League team, as of 1st of April 2019, and the most expensive.

Jordan Pickford, at under £5 million pounds is the cheapest player in the squad, but the total costs in excess of £133m pounds and can therefore not be assembled with the budget of approx. £100m (depending on performance to date).
The most expensive teams is then considered and presented below. This team costs more than £135 million.

1 Goalkeeper, 3 defenders, 4 midfielders, and all 3 strikers (total of 11) make both lists.
The 4 players that could be considered as "under performing" based on their Fantasy Football price are David De Gea, Cesar Azplilcueta, Kyle Walker and Alexis Sanchez.
It should be noted that Romela Lukaku has an identical price to that of Pierre-Emerick Aubameyang.