Why does it cost so much more to build in New York City — far more than any other city? A lot of it has to do with what a new lawsuit charges is the age-old practice of union “corruption from the bottom up.”

Related Companies charges that widespread misconduct by the Building and Construction Trades Council of Greater New York has jacked up costs at its massive Hudson Yards project by $100 million.

As The Post reported last week, the most outrageous scam is the use of two veteran journeymen, rather than the most junior apprentice, to fetch coffee and snacks for workers on high floors.

This menial task was their only job — and it paid them $69.87 an hour, including benefits, and up to $93.86 for overtime.

One “coffee boy,” described as the 55-year-old brother of a high-ranking union official, put in for 155 hours of “work,” 45 of them on overtime, in a single month. Another worker claimed 12 hours a day, 7 days a week, on the job for an entire year, for $600,000 in pay.

According to the suit, such undeserved hourly inflation is common — up to 20 percent of all claims. And that’s not counting the number of union-mandated “no show” jobs, or the higher-than-allowed hiring of more expensive journeymen.

All this — and more — violated a project labor agreement the union signed, promising to supply the highest-quality labor willing to do an honest day’s work for an honest day’s pay. (The union claims the suit is a “retaliatory response” to its activities.)

Such corrupt practices are nothing new. They constitute the “old-fashioned way” of doing business, the lawsuit charges, and have persisted over several decades.

It gets even more obscene on public projects: The MTA in 2010 discovered that it was paying 200 extra workers, who had no actual duties, (out of 900 total) about $1,000 a day each on the East Side Access project.

It’s one thing for labor to organize — another for it to rob everyone else blind.