Small businesses have started firing back at the credit card companies.

Five million retailers have leveled an antitrust suit against Visa, Mastercard, and 13 large banks over the 2 percent “interchange” fees they charge retailers on credit card transactions. The outcome could cost the defendants billions of dollars and force them to lower fees. Talk about a boost for the economy.

It’s clear that the financial institutions are more than a little nervous about this. Visa has already put more than $4 billion in escrow to deal with the case, but the banks’ possible liabilities are much, much larger than that. Brian Keane, an analyst for Deutsche Bank, estimates the damages could run into the hundreds of billions of dollars.

And that’s only one of the big shoes that may drop. The other is the very real possibility that a judge would order the interchange fees lowered from 2 percent to 0.5 percent – which is the current rate in Australia and higher than the 0.3 percent rate charged in Europe, according to a report by Sanford Bernstein analyst Rod Bourgeois. This would have a larger impact than the Durbin Amendment, which put a cap on debit card fees.

A Deutsche Bank study estimates that reducing credit card interchange fees by 75 percent would cost U.S. Bancorp about $1.2 billion in revenue this year, four times what the Durbin Amendment will cost the bank. For JPMorgan, the estimated cost of reducing credit card interchange fees would be five times that lost to Durbin.

The case, which sadly won’t begin until September, will be heard by Judge John Gleeson of the U.S. Eastern District. Gleeson is no stranger to these issues. He ruled against Visa and MasterCard in a 1996 class action lawsuit led by Wal-Mart and Limited Brands which resulted in the largest antitrust settlement in history. That case cost the card companies $3 billion in monetary damages, and they lost more than $25 billion in revenue due to changes in business practices.

Banks and credit card companies are also facing a challenge in Europe around this. The European Commission is seeking public input on the issue of how to encourage the card companies to reduce rates while increasing the security of payments. One specific question the EC is asking: Why hasn’t the cost of paying by card dropped in the last decade, despite the massive increase in volume of such transactions?

Monopoly – it’s not just a board game.