Well you're already mid victory lap after reading SEPH's unsourced post and seemingly without looking up any of the information.
Reported agent fee's from past summer/winter windows*:
- Chelsea £75.1M (18.8% of total transfer value)
- Man City £60.6M (27.3%)
- Man United £34.1M (19.7%)
- Liverpool £31.5M (21.4%)
- Arsenal £24.8M (12.4%)
*Approximate %s, as I had to convert transfer values from euros to pounds.
I'm no expert on how agent fees are determined in transfers, but I would assume they are somewhat proportional to the total transfer value. Free transfers could throw that off. But Chelsea's % of agent fees to total transfer value isn't out of the ordinary.