The Australian Christian Lobby and the senator Eric Abetz have rejected modelling suggesting the “true cost” of the same-sex marriage plebiscite is $525m.

The accounting firm PricewaterhouseCoopers Australia (PwC) calculated the real cost of the plebiscite at $525m, based on its estimates of $281m for the time cost of voting, $158m to conduct voting, $66m for campaigning and $20m for mental health harms to 50,000 LGBTI Australians.

An Australian Christian Lobby spokesman said PwC was not a neutral organisation because it was a public supporter of same-sex marriage and Australian Marriage Equality, who are working on the “yes” case for the plebiscite.

“These costings fail to take into account the value of a people’s vote,” the spokesman said. “It is valuable to democracy and valuable to the Australian people who mostly support the plan.

“They also fail to consider the enormous cost of the nearly 20 times this issue has been raised in parliament in recent years. Parliamentary time, committees and inquiries are very expensive.”

A plebiscite was “an important way to finally the resolve the matter in a democratic way”, he said.

Abetz told ABC’s AM program the modelling was “a ‘study’ in inverted commas with an outcome that was sought by those doing it”.

He said the modelling measured time taken to vote, which was “not an actual financial cost but just something foregone” and “quite a bizarre way to undertake such an analysis”.

PwC estimated the largest cost was $281m for the one hour it would take 15.5m Australians to vote on a Saturday or 30 minutes to submit a postal vote.

Luke Sayers, the chief executive of PwC Australia, said the findings showed a standalone plebiscite on marriage equality was a “massive waste of time and money” and backed a parliamentary vote as the best mechanism to minimise the cost to the economy and health and wellbeing of the community.

The modelling estimated a parliamentary vote without a plebiscite would cost just $17m, $508m less than a standalone plebiscite.

PwC partner Suzi Russell-Gilford told Guardian Australia the firm “takes diversity very seriously” and was one of the first major corporations to sign a letter supporting marriage equality.

But Russell-Gilford stood by the figures, saying “we thought it was important to find the true cost of a plebiscite in terms of a dollar value”.

PwC was Australia’s top LGBTI employer in 2015, as ranked in the Pride In Diversity Australian Workplace Equality Index.