Legal aid for multiple-murder accused David Bain is rapidly closing in on $1 million, already topping the previous highest amount paid out.

Bain supporter Joe Karam estimates the cost of a scheduled retrial will exceed $10 million, including the cost of a team of up to 30 police working fulltime for the past two years; three Court of Appeal hearings, so far involving four lawyers and three judges; 50 pre-trial applications and the retrial.

"It's astronomical," Mr Karam said. "I'd say by the time it's finished, from the time the retrial was ordered, not counting all of the previous 12 years of carry on ... I think the defence costs alone will be $2 million."

Bain served 13 years for the 1994 murders of his mother, father, brother and two sisters. Last year, his lawyers travelled to London and successfully petitioned the Privy Council, which ruled that there had been a substantial miscarriage of justice and ordered a retrial. Bain was freed in May to await retrial.