One hundred billion pounds a year?!
That's nearly three times more than we currently spend on defence, more than the education budget and only a bit less than the annual cost of the NHS.
In fact, their spending plans are perfectly credible, if you look at the stark realities of the economics of climate change.
It's the cost of replacing every gas boiler, properly insulating every home, and providing infrastructure for cleaner transport.
The proposal met near unanimous support in the House of Commons and all the major parties back the target or more ambitious versions of it.
The Committee on Climate Change - which holds government to account on climate change - projects that reaching net zero by 2050 would cost 1-2% of GDP so up to £50 billion a year.
And, it warned, the longer that spending is delayed, the greater the annual costs in will be in future.
The government's own estimate was up to £70 billion to wean us off fossil fuels.
The Green Party wants to hit the net zero target earlier - in 2030.
Labour has committed to the same target. And doing that will cost more money on an annual basis than the 2050 proposals we've already signed up to.
'We will invest to protect the future of this planet' says Green Party deputy leader Amelia Womack during campaign launch
Whatever party you vote for, the price tag for decarbonising our economy is mind-blowing.
But the headline figure ignores the massive savings it will bring.
Energy bills will dramatically fall, so should spending on transport fuels.
Cleaner air and warmer homes will bring direct savings to the NHS.
New industries from renewable energy to green construction, should generate wealth to compensate for some of the spending.
Polling shows more people than ever will be going to vote on December 12 with worries about the environment.
The challenge for the Green Party is whether voters are prepared to look beyond the spending and accept the benefits of mending our environment.