The Belgian international completed a permanent switch to Everton for a club record £28million in the summer after a successful season on loan at the Goodison club.
Martinez believes Lukaku's qualities - that are unique to the Everton squad - make him value for money, despite the huge outlay that dwarfed the previous transfer record of £15m for Marouane Fellaini back in 2008.
"Remember that the assessment of Rom is what he brings to the side. Nobody else in our squad can give us what he gives us," Martinez said.
"The fee is what he means to our squad and you don't get that sort of player easy.
"The reality as well is that after the World Cup it's been difficult for him to get his best physical level and we need to help him towards that but I've been delighted with what he's done, although I still know he has loads to do because his potential is immense."
For the big stories and the banter follow us on Twitter: @tribalfootball