The Belgium international forward arrived from Stamford Bridge for a club-record transfer fee of £28 million earlier this summer in a stunning deal that significantly dwarfed the £16m they paid to land Marouane Fellaini from Standard Liege back in 2008.
Lukaku spent last season on loan with the Toffees and impressed as he finished top of the club's scoring charts with 15 goals, as thge Blues pushed Arsenal for the final Champions League spot.
Martinez's side eventually finished fifth, and the Spaniard made Lukaku his number one priority in the transfer market this summer.
Everton are often seen as one of the less wealthy sides at the higher end of the Premier League, and the blockbuster deal to land Lukaku caught several football followers by surprise. But the Toffees coach believes that he got value for money by signing the 21-year-old.
He told reporters at a press conference: "It's not about the value, I would have paid £100m for Lukaku. If I had the money, yes, why not? 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 is expected to be in the side that play against Chelsea on Saturday evening, and Martinez is not expecting his big-money buy to approach the game against his former club any differently to a normal match in the Premier League.
"He is not going through the motions, he is raising his levels for every game," he added. "Every game he is fresh, so I do not see any difference at all for this game."
Everton have also signed Muhamed Besic, Gareth Barry and Samuel Eto'o, while Christian Atsu has arrived on loan from Chelsea. However, the Ghana international winger is unavailable for Saturday's game.