10. Romelu Lukaku. (15M£)
Romelu Lukaku is considered as one of the brightest young talents in the Europe currently and Chelsea surely had spotted the talent in him back then, when he was still 18 years old and the Blues paid 15M£ for the Belgian forward. Romelu Lukaku then had some loan spells under West Ham and Everton for whom he did quite well, but failed to get noticed by Jose Mourinho. Chelsea currently don’t really need the Belgian to add up to their attacks, but the amount that they paid for him makes him the 10th most expensive teenage transfer ever.