That cost includes the veterinary treatment and care of the rescued horses.
A still from the World Horse Welfare video, below.
The Grays lost their appeals against conviction early today and head of the family James Gray faces a recall to prison to serve a 24-week sentence if he is unsuccessful in his appeal against the sentence, which will be heard next week.
He also faces court costs which stand at £400,000.
Judge Christopher Tyrer, in a judgment delivered early today (NZ time) in Aylesbury Crown Court, said the Grays had taken no action to alleviate the suffering of the animals in their care.
World Horse Welfare vet Andy Williamson, Nicolas de Brauwere of Redwings Horse Sanctuary, and vet Peter Green, who all gave evidence for the RSPCA, were praised by the court for their compelling testimony.
The family was prosecuted after RSPCA inspectors discovered more than 100 horses, ponies and donkeys and the bodies of a further 32 equines at Spindle Farm, near Amersham, in January 2008.
RSPCA inspector Kirsty Hampton, who investigated the case, described the conditions that the horses were kept in as "grotesque".
World Horse Welfare field officer Nick White, who describes the conditions at Spindles Farm in the video below.
"The stench of decomposition and urine was overpowering. The sight of horses left in such a miserable state will stay with me forever and I hope I never have to see animals treated with such little care and compassion again."
Officers from World Horse Welfare, Redwings Horse Sanctuary, vets and Thames Valley Police worked alongside the RSPCA in difficult conditions to rescue the surviving animals from the scene.
Many of the animals had little food or dry bedding and were crammed into pens, ankle deep in faeces.
Other horses had simply been left to die where they fell and then, surrounded by their companions, decomposed. Further carcasses were discovered in surrounding fields, some burned on bonfires, and there was a pile of bones and a skull against an outbuilding.