About £45m was spent by Transport for London (TfL) making more than 800 people redundant over the controversial plan to close Tube ticket offices.
BBC London has learned that 823 staff took redundancy while 54 left for other reasons such as retirement when the offices closed in 2015.
The TSSA union said it was a waste of money as 325 jobs were reintroduced following a review.
The cost was "significantly less" than the savings that were made, TfL said.
The plan, announced in 2013 by then mayor Boris Johnson, has prompted numerous strikes.
TfL wanted to close the offices to help pay for the 24-hour Night Tube service which is marking one year in operation this Saturday.
When Sadiq Khan became mayor, he ordered London Travelwatch to carry out a review which found that staff were not visible enough and more were needed.
Ben Stevenson from the TSSA, said: "As a union we didn't want anyone to be made redundant as it is one of the most stressful situations anyone can go through."
He said it was right staff were paid what they were legally entitled to but that too many people left the company.
"Our objective was to get as many jobs back as possible to ensure the safety of the public and the service," he added.
TfL said it had always planned to review staffing levels following the ticket office closures.
A TfL spokesperson said: "The one-off voluntary severance costs associated with the closure of ticket offices in 2016 are significantly lower than the hundreds of millions of pounds we will be saving across our business plan from these changes.
"We are now undertaking the biggest overhaul in our history which will save £4bn over the next few years and enable us to deliver a modern, affordable and accessible transport network for all Londoners."