The Liverpool boss hit back after Fergie claimed £20million Kop striker Robbie Keane was overpriced.
Berba cost Manchester United £30.75m. Benitez said: “Alex should think about how much they spend every year.
"Maybe he feels Berbatov was cheap because they pay £30m for a lot of players and £20m for the majority of them.
“In our last game against them they had Owen Hargreaves, Carlos Tevez and Nani on the bench — that’s £70m.
“But if I leave Fernando Torres or Keane on the bench, people are asking questions.
“If they leave those three on the bench you don’t hear anyone saying anything.”
Benitez added: “If you talk of the price of players we can’t match United because their strikers cost £60m and ours cost £40m.
“He spends £30m each on three or four players every two or three years. And anyway, our strikers are really good.
“Yes, I was a bit surprised to hear him talking of Keane’s price but maybe that will be the added motivation for Robbie.
“We’re only talking about it because they were talking about us.
“If Keane scores goals and does really well for us then the price doesn’t matter.
“And if you talk of what we’ve brought in, it’s been big money over the last year or so. Analyse the last five, six, maybe 10 years and look at the value of our squad now.”
Since Benitez took over at Liverpool in June 2004, he has spent £188m on 26 players.
During the same period, Fergie has spent £177.5m on 14 players.