Everton star's inconsistency criticised.
The former England international believes Lukaku needs to become more like Chelsea striker Diego Costa if he is to live up to his potential.
Everton's Romelu Lukaku celebrates scoring their second goal
Referring to numerous previous media reports regarding Lukaku's suggested price tag, Redknapp said: "I look at Romelu Lukaku and struggle to put a value on him.
"There are times when he plays like a £60million centre forward and others when I would not pay £10m for him.
"He should be modelling himself on Diego Costa. Lukaku is a battering ram of a forward who needs to be more aggressive and bully defenders."
Lukaku scored two goals in the weekend loss to Watford, but before that he had gone five straight games for club and country without netting.
This season he has nine goals in 14 Premier League appearances, with a further five for Belgium in friendlies and World Cup qualifiers.
Everton's Romelu Lukaku scores their second goal
Lukaku began his career at Anderlecht before moving to Chelsea as a youngster, and leaving for Everton after failing to hold down a starting spot.
Still just 23, he has ambition to play for a Champions League level club, but he needs to cure his inconsistency and become a 25 goal a season player to demonstrate he is worth the high price Everton would demand for his signature.