For me the best transfer for major clubs like Barca/Madrid/United/City/Chelsea/Milan cannot be judged in terms of value for money.
A top club, requires world-class skills, such as those provided by Silva, Aguero, Cesc etc. not a high return on investment.
In other words, a top club is happier if it plays 60m for a player that does the work of a 60m-player, and not if it pays 5m for a player that does the work of a 20m-player.