The tie-up agreement between NYSE Euronext and Deutsche Börse would cost a rival bidder about $339m to break up the deal, according to documents filed with regulators.

Both NYSE Euronext and Deutsche Börse agreed to pay the other party a €250m termination fee should the sector-reshaping exchange deal be spoiled, as speculation arose this week around the likelihood of CME Group mulling a counteroffer for Big Board parent NYSE.