Boeing upped its wage proposal to thousands of striking workers today, offering a 30% general wage increase over four years in what it called its “best and final” offer as the strike drags on.

The U.S. planemaker is also offering to reinstate a performance bonus, improve retirement and double a ratification bonus to $6,000, if the workers accept by Friday.

The Seattle-area Boeing workers who build the planemaker’s strongest-selling 737 MAX jet and other planes assembled in its Washington state factories went on strike on Sept. 13, after rejecting their first full contract in 16 years.

Boeing has frozen hiring and started furloughs for thousands of U.S. employees to reduce costs. A prolonged strike could cost several billion dollars, fraying the planemaker’s already strained finances and threatening a downgrade of its credit rating.

The strike, Boeing’s first since 2008, is the latest event in a tumultuous year for the company that began with a January incident when a door panel detached from a new 737 MAX jet mid-air.

Factory workers rejected an earlier tentative deal between Boeing and the union, which offered a 25% raise over four years, and a commitment that a new plane would be manufactured in the Seattle area if it were launched during the four-year agreement.

A union spokesperson for the International Association of Machinists and Aerospace Workers was not immediately available for comment.