Made in the summer of 2016 based on what was said by Sound Transit back then, and what was known then about the alternatives.

Same as it ever was.

The $54 billion cost estimate back in 2016 put out for ST3 was an incremental cost number reflecting the additional spending needed for the specified phase 3 additions (including the extension to West Seattle) beyond any fully-funded work paid for by the first two phases of Sound Transit taxes approved in the 1996 and 2008 successful tax district elections.  The financial number used now comes from Sound Transit spending and budgets that cover all construction and operations 2017 to 2046, totaling $148.2 billion, including spending to operate phase one (Sound Move), and to finish building ST2 and operate it, plus ST3 construction and operation, and as always, including operating and maintaining Sounder Commuter Rail and the various ST bus services, which now include STRIDE Bus Rapid Transit that cost as much as light rail to build. 

Construction of all ST3 lines is planned to be done by 2046, unlikely, but even if it were to come true, the multi-modal rail and bus operations, maintenance, renovations, upgrades, and loan payments will continue beyond and forever.