The Grenfell tower inquiry has cost the taxpayer at least £40m so far, more than 100 times the savings made by swapping fire-retardant cladding on the council block for cheaper combustible panels that fuelled the fatal fire.

Most of the money went on the array of more than 150 lawyers and legal fee earners representing the inquiry and the bereaved and survivors from the 14 June 2017 disaster that claimed 72 lives.

These QCs, counsel and solicitors have so far charged £24m, according to accounts published by the inquiry on Friday. They include 52 barristers instructed by the inquiry, which is chaired by the retired appeal court judge Sir Martin Moore-Bick, who earns up to £220,000 a year, 13 barristers and 91 other fee earners, mostly solicitors, for survivors, bereaved and residents.

The inquiry was established by Theresa May in August 2017 to look into the events of the night of the fire and the circumstances of the refurbishment project that preceded it. It has spent £4.6m on expert evidence and scientific investigations, £9m on back-office costs and £2.6m on hiring the venue at Holborn Bars in London’s legal district.

The costs do not include well over £10m already spent on legal fees by other public bodies and government departments, including the Royal Borough of Kensington and Chelsea, which oversaw the decision to shave £293,368 from the refurbishment budget by swapping zinc cladding with a fire-resistant core for an aluminium alternative with a combustible polyethylene core. Kensington and Chelsea has allocated £6.3m to the legal costs.

It means the inquiry has already cost considerably more than the Iraq inquiry, chaired by Sir John Chilcot, which spent just over £13m, and the Leveson inquiry into press culture and standards, which cost about £5.4m.

The £18.9m cost of representing the 585 bereaved, survivors and residents who have core participant status does not yet match the £63.6m legal costs for representing the families of the 96 people who died at Hillsborough before and during the two-year inquest that ended in 2016.

But with at least another two years to run, lawyers’ bills are set to soar during the second-phase hearings, which will last an estimated 18 months – three times longer than the first phase.

“You can’t put a cost on justice,” said Deborah Coles, the executive director of Inquest, a charity that provides expertise on investigations into contentious deaths. “The vital role that the families’ lawyers play in going through thousands of documents and making sure that evidence is properly presented and helping them through a complex and difficult process cannot be overestimated. In the next stage there will be unlimited funds for the corporate lawyers so there has to be a level playing field for families”.

Private companies involved in the construction project are already using 20 firms of solicitors, 14 QCs and five other barristers, suggesting that all told the costs of the inquiry could easily exceed £100m when it finally concludes no earlier than late 2021.

The inquiry has received and reviewed more than 500,000 documents, of which more than 200,000 are expected to be disclosed to core participants.

On Wednesday, Moore-Bick published his report into the night of the fire which found the London fire brigade’s readiness was “gravely inadequate”, fewer people would have died if it had been better prepared, and that the refurbishment project that wrapped the building in combustible insulation breached building regulations.

Survivors called for the resignation or dismissal of Dany Cotton, the commissioner of the London fire brigade, but she resisted and claimed Moore-Bick lacked evidence to say fire commanders’ slowness to order an evacuation cost “many more lives”.

In comments that angered the bereaved, she said: “We note the chairman states he has received no expert evidence to guide him on reaching his conclusion.”

That was true, but he went on to say: “I am confident that, on the clear and extensive evidence about the events of the night that I have heard at phase one, I can and should reach the conclusion that … [evacuation should have been ordered at least an hour before it did and] would have resulted in the saving of many more lives.”

Natasha Elcock, the chairwoman of Grenfell United, the survivors and bereaved group, said she saw the conclusion of the first phase as the first of four stages in the survivors’ search for justice. Next comes the second phase of the inquiry into the refurbishment project, which is overlapped by the ongoing Metropolitan police investigation into possible manslaughter, corporate manslaughter, individual gross negligence and health and safety offences which currently involves more than 160 police officers and staff.

Leaders of the London fire brigade have already been interviewed under caution and the police said in June that they had conducted 13 interviews under caution, gathered 45m documents in digital form and 14,500 physical exhibits. Finally, the survivors and bereaved hope for prosecutions, convictions and jail sentences.