MPs are paid £86,584 a year, with an additional £31,680 for those promoted as ministers of state.
It means Mr Freeman would have been paid £118,264 per year before resigning.
But despite the figure making him one of the UK’s highest earners, the Tory MP wrote on his blog that a sharp increase in mortgage payments was part of the reason he stepped down.
“I was so exhausted, bust and depressed that I was starting to lose the irrepressible spirit of optimism, endeavour, teamwork & progress which are the fundamentals of human achievement,” Mr Freeman said.
He added: “And because my mortgage rises this month from £800 pcm to £2,000, which I simply couldn’t afford to pay on a ministerial salary.”
Mr Freeman lashed out at what he called “political economy 2.0”, saying Britain is “in danger of making politics something only Hedge Funder Donors, young spin doctors and failed trade unionists can afford to do”.
Quitting his government role will allow the MP, who spent more than a decade in the life sciences and technology sectors before entering parliament, to take on lucrative jobs outside parliament, as long as they are approved by the Advisory Committee on Business Appointments (Acoba) watchdog.
Interest rates spiked in the UK following Liz Truss and Kwasi Kwarteng’s disastrous so-called mini budget in September 2022.
They have also been pushed up by a series of Bank of England interest rate hikes in a bid to control spiralling inflation.
Signs that inflation is returning to the Bank’s target and that interest rates are likely to begin falling this year have led to a series of lenders offering cheaper mortgages. But those facing renewal this year will still face paying around £400 a month more on average than they were.
Resigning as science minister in November, Mr Freeman said it is “time to focus on my health, family wellbeing and life beyond the front bench”.
The Mid Norfolk MP said he has championed science and technology since he was elected 13 years ago, holding five ministerial posts under four prime ministers.
And in his blog post on Friday he pledged to use the build-up to this year’s general election to “share and inform the policymakers working on the manifestos across Westminster”.
He said he was doing so “in the hope that the next government (whoever forms it) will be able to build on our successes and learn from our failings in delivering this vital mission”.
Mr Freeman also used an interview in the New Statesman to say it is “very likely that we’re going to have a Labour government” after the election.