Formula One's governing body has agreed to exclude drivers' salaries and marketing costs from an optional budget cap to be introduced for teams next season.
International Automobile Federation (FIA) president Max Mosley has also written to teams, in a letter seen by Reuters at the Bahrain Grand Prix, asking them to advise him by close of business on Thursday whether the £30m cap should be revised.
He warned that the sport, which saw Honda depart in December, risked losing at least two more teams for 2010 were the cap not to be an option.
"We are now close to concluding the regulations, both financial and technical, for presentation to the World Motor Sport Council on 29 April," Mosley wrote.
"The financial regulations will be based largely on the principles established by the teams and the FIA last year. The key point is that the budget cap for 2010 will cover all team spending except for marketing and drivers."
Those accepting the budget cap will have greater technical freedom than teams who prefer to continue with unlimited budgets, theoretically allowing them to be competitive with less money.
Several parties, including Lola and former Benetton and BAR boss David Richards, have expressed interest in entering under such a cost cap and Mosley said the FIA would select three.
"This will demonstrate that with less than £30m, it is possible to run a Formula One team which is virtually indistinguishable from an unlimited-expenditure team," he said.
"Apart from the three new teams, we have had indications that some existing teams would like to run under a cost cap in 2010," added the Briton.
"However this might mean an adjustment of the £30m. Could you, therefore, please indicate what you would consider to be a minimum acceptable figure for the cap?"
The cost cap option was put forward as part of measure to cut costs and protect the sport from the ravages of the global credit crunch.
The Formula One Teams Association (FOTA) has said teams will be able to secure an engine and gearbox supply next year for 6.5 million euros
"World economic conditions and the outlook have deteriorated considerably since cost cap targets were discussed last year," said Mosley.
"As a result, it is likely that without the opportunity to participate under a cost cap, we will lose at least two further teams for the 2010 season, notwithstanding the cost-saving measures which FOTA has already agreed.
"This would leave us with just 16 cars," he added.
"However, the FOTA cost savings have been achieved at a pace which suggests that teams can adjust much more quickly than was apparent 12 months ago."