Mercedes is not opposed to the introduction of a budget cap in formula one but "the challenge is how to control spending".
In Melbourne, FIA president Jean Todt said preparations for "cost control" are "quite well advanced".
He said the plans will be presented by the governing body and Liberty Media to the teams in London on March 26.
But top teams Ferrari, Mercedes and Red Bull-Honda remain opposed to several details of the 2021 plan.
"If it goes in the wrong direction, Dietrich Mateschitz could pull the plug," Red Bull’s Christian Horner warned, according to the Swiss newspaper Blick.
And Toto Wolff, the boss at Mercedes, suggested the biggest concern is the proposed budget cap.
"Tthe budget cap is still a work in progress but there will be a budget cap. We want it," he said.
"The difficulty is the controllability of the expenses, because the teams are structured very differently. For example, we will need to agree penalties in the event that a team deliberately breaks the rules.
"It is very complex to put all that in place. We see the biggest challenges with the implementation," he added.
"Formulating everything is very complex," Wolff added.
F1 chief executive Chase Carey, however, said Liberty Media will try to agree "compromises" with the dissenting teams.
"There is a general direction we are going with everything, and then there are the details, and there are ten different views on the details," he said in Melbourne.
"It is not unique in the world to try to find compromises."