After a successful season on loan at Goodison Park, the Belgium international made the permanent switch to the Toffees this summer, leaving Chelsea in a £28m deal.
And the Spaniard is adamant that he has not overpaid for the striker, arguing that securing the right personality to take the team forward can be priceless.
|11/2||Romelu Lukaku is 11/2 with PaddyPower to score in an Everton win against Chelsea|
“The figure is insignificant. It is about the character and type of player you want in the group.
"Sometimes a player at £3m can be an expensive buy and sometimes a player at £28m can be a very cheap buy."
Lukaku will face his old club when Chelsea go to Merseyside on Saturday but Martinez does not believe that the chance to prove a point to Jose Mourinho will affect how seriously the striker takes the match.
He remarked of the idea: "I know what you are saying. My answer is, of course, he is human and, leading up to the game, people will say 'you are facing your old team-mates and manager'. That's normal.
“But what I am saying is that his level of performance will not change from against Arsenal or Leicester.
"He is not going through the motions, he is raising his levels for every game. Every game he is fresh so I do not see any difference at all for this game."