Benitez suggested last week that Ferguson had been far busier in the transfer market but the United boss has done his own calculations and believes the Spaniard is way ahead since he took charge at Anfield five years ago.
Ferguson also expects Benitez to make several signings this summer after putting pen-to-paper on a new contract.
"I was amazed at that one," said Ferguson. "I got some of the people in the sports technology department to check that out.
"He's well ahead of us in spending in the last five years.
"He said our spending power is more than theirs but he has spent £24million more than we have.
"I think you will see Rafa producing an absolute spending spree, I think that's an absolute certainty now he has signed a new contract.
"There's talk about a recession but I don't think there will be one at Liverpool this year."
BalanceIn terms of net spending - taking into account income from sales - the clubs are much the same. Liverpool's net spending over five years on players is £82.5million and United's has been £85.5million.
But Ferguson insists he is also committed to bringing through younger players, while Benitez is more focused on bringing in experienced professionals.
He explained: "There's a balance - we have signed 18 players in the last five years and eight of those have been young players. There's a balance, it's about doing the thing correctly.
"We like to try to develop young players and we will always do that. That's the nature of the club.
"Other clubs are maybe different and obviously Rafa has a different philosophy of the game from mine.
"But that's the great thing about football - everyone has a different way of doing things.
"The most amazing fact was that they have used 60 players in reserves matches and have 50 players in their reserves squad."