The war in Afghanistan has cost Britain at least £37bn and the figure will rise to a sum equivalent to more than £2,000 for every taxpaying household, according to a devastating critique of the UK's role in the conflict.

Since 2006, on a conservative estimate, it has cost £15m a day to maintain Britain's military presence in Helmand province. The equivalent of £25,000 will have been spent for every one of Helmand's 1.5 million inhabitants, more than most of them will earn in a lifetime, it says.

By 2020, the author of a new book says, Britain will have spent at least £40bn on its Afghan campaign, enough to recruit over 5,000 police officers or nurses and pay for them throughout their careers. It could fund free tuition for all students in British higher education for 10 years.

Alternatively, the sum would be enough to equip the navy with an up-to-date aircraft carrier group, or recruit and equip three army or Royal Marine brigades and fund them for 10 years.

In the first full attempted audit of what he calls Britain's "last imperial war", Frank Ledwidge, author of Investment in Blood, published next week by Yale University Press, estimates British troops in Helmand have killed at least 500 non-combatants. About half of these have been officially admitted and Britain has paid compensation to the victims' families.

The rest are based on estimates from UN and NGO reports, and "collateral damage" from air strikes and gun battles.

Ledwidge includes the human and financial cost of long-term care for more than 2,600 British troops wounded in the conflict and for more than 5,000 he calls "psychologically injured". Around 444 British soldiers have been killed in the Afghan conflict, according to the latest official MoD figures.

The MoD has estimated the cost so far of conducting military operations in Afghanistan to be about £25bn. MoD officials said on Wednesday that British troops were in Helmand to protect British national security by helping Afghans build up their own security forces.

The ministry does not keep figures on civilian casualties and has told the Commons defence committee that it cannot provide a figure for the "total" cost of operations in Afghanistan.

Ledwidge, who has also been a civilian adviser to the British government in Iraq, Libya and Afghanistan, says Helmand is no more stable now than when thousands of British troops were deployed there in 2006. Opium production that fell under the Taliban, is increasing, fuelling corruption and the coffers of warlords.

"Rendering the Afghan armed forces capable of securing the province [Helmand] is regarded by many ordinary British soldiers as little short of ridiculous," Ledwidge writes.

Though British and other foreign troops were sent to Afghanistan to stop al-Qaida posing a threat to Britain's national security, "of all the thousands of civilians and combatants, not a single al-Qaida operative or 'international terrorist' who could conceivably have threatened the United Kingdom is recorded as having been killed by Nato forces in Helmand," Ledwidge writes.

The real beneficiaries of the war, he suggests, are development consultants, Afghan drug lords, and international arms companies. Much of British aid to Afghanistan is spent on consultancy fees rather than those Afghans who need it most.

It was a serious mistake, the author adds, to treat al-Qaida as a military problem – the problem was primarily an intelligence one. Reflecting the widespread view across Whitehall and among defence chiefs, he says the real reason Britain has expended so much blood and money on Afghanistan is simple: "The perceived necessity of retaining the closest possible links with the US."

Ledwidge told the Guardian: "Once the last British helicopter leaves a deserted and wrecked Camp Bastion, Helmand – to which Britain claimed it would bring 'good governance' – will be a fractious narco-state occasionally fought over by opium barons and their cronies."

He added: "There are no new lessons here, only one rather important old precept: before you engage in a war, understand the environment you are going into, precisely and realistically what it is you are trying to achieve and will it be worth the cost? In other words have a strategy."