It’s a simple question and the fact NDP Leader Tom Mulcair won’t answer it speaks volumes.

How much will his plan to impose a national cap-and-trade carbon pricing scheme on Canadians cost?

The late Jack Layton, former NDP leader, gave an answer to voters in his 2011 campaign.

He said it would increase government revenues by $21.5 billion over four years.

In Ontario, where Premier Kathleen Wynne will introduce cap-and-trade in 2017, government bureaucrats have pegged the cost at up to $2 billion annually, which Ontarians will see in higher prices for most goods and services.

Finally, Mulcair himself has talked in the past about how cap-and-trade would bring in billions of dollars in new government revenues for the NDP, without citing a specific number.

Indeed, cap-and-trade appears to be one of the major revenue sources Mulcair plans to use to pay for his expensive campaign promises, such as one million, publicly-subsidized daycare spaces costing parents $15 a day.

Asked about it on the campaign trail, Mulcair’s answer was a study in gobbledygook about attending yet another United Nations meeting on climate change in Paris later this year and promising to work with the provinces.

But no reference to the costs.

Mulcair described cap-and-trade as “the best way of ensuring that there is a guaranteed reduction” of industrial greenhouse gas emissions, which is not supported by real-world evidence.

The largest cap-and-trade market in the world, Europe’s decade old Emissions Trading Scheme, has been an abject failure.

All it did was raise electricity and other consumer prices across Europe, while its impact on emissions has been insignificant.

That’s in part because the market is overrun with fraudulent carbon credits, for which no actual reduction in greenhouse gas emissions occurred.

To be sure, Mulcair isn’t the only Canadian political leader to propose cap-and-trade.

Aside from Layton, former Liberal leader Michael Ignatieff also ran on it in the 2011 election and Harper proposed it during the 2008 campaign, when it looked like the Americans were about to do it as well.

So it’s not a new concept. Even Mulcair ought to be able to give a straight answer by now.