Roberto Martínez has claimed Romelu Lukaku’s value to Everton cannot be measured by his £28m fee and denied the price is excessive compared with the £32m Chelsea spent on Diego Costa.

Lukaku will face his former club, Chelsea, at Goodison Park on Saturday providing he overcomes a toe injury. Another former Chelsea striker turned Everton signing, Samuel Eto’o, could also be included against José Mourinho’s team depending on match fitness.

Everton paid a club-record fee to secure Lukaku while Costa cost Chelsea an additional £4m from the Spanish champions Atlético Madrid. Martínez, however, believes the fees reflect the reality of the transfer market and Lukaku’s importance to how his Everton team operate.

The Everton manager said: “They are different ages [Lukaku 21, Costa 25] and the situation at Atlético Madrid is very different in how they work. Sometimes as a club you have a say and sometimes you have investment groups who have a say. That could be one reason.

“Second, it’s not about the value. I would have paid £100m for Lukaku if I had the money. 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.”

Mourinho wished Lukaku well when he left for Everton, with the striker returning the compliment, before expressing doubt over the forward’s willingness to fight for a first-team place at Chelsea. Martínez does not expect a change in attitude from Lukaku should he face his former club. “Leading up to the game people will be talking to him about facing his old team-mates and manager, that is normal,” Martínez said. “But his level of performance will not change from against Arsenal or Leicester. He has a status and a vital role for us.

“He wanted to play 90 minutes against Leicester [in the opener] when he was not ready and wanted to stay on with an injury against Arsenal [last Saturday] when it would be easy for him to say he wanted to be ready for Chelsea and so wait until he is 100%. He is not going through the motions, he is raising his levels.”

Martínez suggested Mourinho’s willingness to sell Lukaku stemmed from the need for proven rather than developing talent at Chelsea. “You want to play a certain way and certain types fit in better than others. We let players go as well. That is the nature of the game. Some managers give players time, some managers know they can not give them time. That is football.”