Gov. Cuomo left unfinished business behind in Albany last week. But there’s one piece of business Cuomo hasn’t even started: finding money for the MTA.

Yeah, it’s just as much an emergency as rent control. If Cuomo doesn’t come up with a plan now, he’ll cost us more later — as delays lead to higher costs on multi-billion-dollar megaprojects.

Cuomo has always been in the clouds when it comes to what’s underground.

Last year, he said the MTA’s plan to invest $32 billion in subways, buses and commuter rail over five years was “bloated” — and then he piled on a few more empty pounds.

The MTA can’t afford to finish the projects it’s already begun — the Second Avenue Subway plus East Side Access to bring the Long Island Rail Road to Grand Central.

But Cuomo also wants to build a new train to La Guardia Airport.

Leaving town with a bunch of unfinished, unpaid projects isn’t just . . . disorganized. It’s expensive, too — like when you don’t pay your credit-card bill on time, and owe a $40 late fee.

Only this time it’s tens of millions extra — at least.

Consider East Side Access, a new Grand Central under Grand Central, for the trains used by Long Islanders.

When then-Gov. George Pataki, then-Sen. Al D’Amato and, later, Sen. Chuck Schumer OK’d this thing starting nearly two decades ago, it was supposed to cost $3 billion and be done by 2012. Now, it’s costing $10.2 billion — and it’ll be done by 2022. If we’re lucky.

Don’t say we weren’t warned.

Back in 2000, when a Newsday columnist worried East Side Access would be “one of the greatest boondoggles in New York history,” then-LIRR chief Tom Prendergast conceded that “it will cost a lot of money,” but insisted that it was a terrific idea.

Now, Prendergast is in charge of the whole MTA — and in charge of finding that money.

Is East Side Access a good idea or not? As Hillary Clinton might say: What difference, at this point, does it make?

The project is half built — and more than half paid for.

“They’ve made us a hole,” said one construction manager. “It’s not fair to the public to leave us with an open hole.” (To be fair, the hole is pretty cool, and they’ve built some tunnels, too.)

It’s Cuomo’s open hole now, though — and it’s time to just get it fitted out with escalators and tracks and the like, and do it fast and do it right, so that 80,000 Long Islanders can use it every day.

But to keep the project on its already-way-late schedule, the MTA has to award two massive contracts — each “over $100 million” in its words, but probably multiple times that — to finish out the Manhattan caverns and complete the tunnels and tracks in Queens.

The problem, however, is that the MTA doesn’t have the money.

Most of the money the MTA must spend on heavy-duty investments over the next five years isn’t for new projects like this one or the Second Avenue Subway, but just for replacing and repairing the old stuff — C train cars, for example — we already have.

The MTA can’t keep building new stuff until it’s taken care of the old.

But the MTA has cobbled together only $18 billion out of the $26 billion it needs for the old stuff. It can’t even think about spending more money on a new project until it finds the rest of the money for the old.

To finish East Side Access on time, though, the MTA has to award these contracts. And it needs to do so right now.

In January, our federal overseers said that doing the caverns is “path critical” — meaning you can’t do much else unless you do this right.

The feds further warned that “the timing and availability of funding presents a significant schedule risk to the project.”

Even if the MTA awarded the caverns contract next month, work wouldn’t start until April, and be done until late 2019 — with more work on other contracts after that.

So delays really do put that 2022 “finally done” date in jeopardy.

And time is money — because construction labor costs go up. “This is not like buying TVs,” said one construction guy. “They don’t get cheaper.”

With Cuomo taking off soon for the summer with no way to pay for this stuff — and seemingly no clue that the problem exists — he’s becoming yet another governor who erodes the public’s confidence that government can build stuff right.

The Tea Party ought to send him a thank-you note.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.