Transfermkt can only be used as a guideline and isn't a 100% accurate site as far as stats are concerned. Sometimes they are accurate, other times they aren't.
Kevin Carlos market value around 4.5m euros, transfer fee around 6m euros to sign for Nice from Basel
Willem Geubbels market value around 10m euros, transfer fee 9m euros to sign for Paris FC From St.Gallen.
Both players were aged 24 at time of transfer.
Realistically, would Geubbels have been our WOW transfer at 9m euros or would it have been Carlos at 6m euros?