The US wants enough Cipro to treat 12 million people
The treatment is made by German pharmaceutical firm Bayer, which has confirmed it will sign an agreement with the US authorities on Wednesday.
The two sides held talks on Tuesday amid pressure from the US Congress to disregard Bayer's patent and buy ciprofloxacin, the generic name for Cipro, cheaper elsewhere.
Late on Tuesday, US health secretary Tommy Thompson said he had won agreement from Bayer to slash the price of Cipro to below $1 per tablet.
Cheaper than Canada
"We're going to be under one dollar, I can assure you," Mr Thompson told the "Larry King Live" television programme.
Bayer president and chief executive Helge Wehmeier confirmed: "We came to an agreement in principle".
A spokesman for the German firm, Michael Diehl, declined to confirm the new price, saying only that said the firm will sign an agreement with the US health authorities.
Cipro's wholesale price in the US is about $4.60 a tablet, while the price for the US government prior to Tuesday's agreement was $1.80.
Mr Thompson said the US will now get the tablets cheaper than the Canadian authorities, which have been pursuing their own talks with Bayer.
Massive shortfall
The US needs to buy vast quantities of Cipro to stockpile enough to treat 12 million people for 60 days - the official target.
It currently has enough of the antibiotic to treat 2 million people.
Bayer has switched to 24-hour production of Cipro since letters contaminated with anthrax started to appear in the US postal system.
So far, three people have died of anthrax and 12 cases have been confirmed.
Staff at Congress, the postal service and news organisations who are thought to have been exposed to anthrax spores have begun taking Cipro as a precaution.