[That's Lawrence Tribe around noon.]
The background document [pdf] says:
EPA estimates that the costs of compliance would range from $5.5 billion to $7.5 billion in 2020 and from $7.3 billion to $8.8 billion in 2030. According to other estimates, however, the potential costs could be much higher.
Footnote 9 says:
See, e.g., NERA Economic Consulting report entitled “Potential Energy Impacts of the EPA Proposed Clean Power Plan,” October 2014.
The NERA study, prepared for an assortment of energy industries, finds that the present value of costs would range from $366 billion to $479 billion from 2017 to 2031. This is roughly about twice the EPA estimates.