Mayor Bloomberg’s latest ed-reform plan has gotten much attention.

To get the teachers’ union to support merit pay, Bloomberg proposes to give teachers a salary hike of $20,000 if they prove “highly effective” for two years in a row under a new teacher-evaluation scheme.

As the mayor put it, “we’ll … work to retain the best teachers — by offering them a big raise.”

As the mayor notes, this would be a huge jump — 26 percent above the $78,000 a teacher makes after a decade on the job. And teacher salaries are already up 43 percent under the decade-old Bloomberg administration.

And this boost would be permanent. “Salary” means it’s not a one-time bonus. Even if they’re not “highly effective” later, they would keep getting this money, forever.

This carrot is one meant for the teachers — but taxpayers could end up paying a lot for it.

The city would determine who is “highly effective” through a ratings system based on “measurable improvement in student performance and principal assessment.”

But the kinks are hardly out of the systems we already use to evaluate student performance. Creating a good system to figure out how much teachers add to student performance, and to rate them, is difficult and unproven.

It’s easy to envision a system that rates all teachers as highly effective, or one that is inconsistent, allowing all teachers to reach the high-water mark for at least the two years they need to score their higher salaries.

After all, the city rates restaurants, and the grades outside the windows often change.

Moreover, it’s hard to see the teachers’ union agreeing to a system that didn’t offer all teachers a good shot at the top score.

And the union has the upper hand, as Bloomberg lost on his signature effort last year — changing “last in, first out” rules for layoffs — and wants a victory now.

In any case, it wouldn’t look good for the city if most of its teachers weren’t “highly effective.”

Think of the numbers here. New York has 88,721 teachers. Say half of them were rated “highly effective” — a very conservative guess. Awarding half of teachers a permanent $20,000 raise would cost $887 million.

That’s nearly a billion dollars in extra spending, every single year. Plus, eventually, teachers retire, and they would get higher pensions based on those higher salaries.

Speaking of pensions, it’s not clear how the new approach to teachers squares with last year’s promise:

I will not sign a contract [with a municipal union] with salary increases unless they are accompanied by reforms in benefit packages that produce the savings we need to continue making investments in our future and protecting vital services, period.

The mayor has kept that pledge, and should continue to do so.

But it’s hard to see the teachers’ union agreeing to a salary hike that requires them to take (or appear to take) a risk, and then give much of it right back in the form of benefits reform.