Last week Friends of the Earth Scotland (FoES) staged an on-site demonstration as it unveiled figures showing that the Scottish Executive has agreed to pay around £100 million in compensation to companies that will be forced to relocate.
FoES chief executive Duncan McLaren commented: "We are now exposing the true cost of the folly. Scottish taxpayers are being fleeced."
Earlier this year FoES and lobby group Jam74 mounted a legal challenge to the executive's decision to reject the findings of a public inquiry and press ahead with the £500 million extension.
Ministers found that the inquiry reporter failed to give sufficient weight to the project's potential for regeneration. The court case is expected to be heard next June.