This is one of those things that, I suspect, was carried over from the new construction-oriented rating systems without much consideration as to how it applies to EBOM. There really is no specific methodology - I have seen projects use figures ranging from only the dollars spent on external consultants, to a comprehensive figure integrating all relevant capital spending, internal hours, etc... One useful way to approach this might be to make use of the IOc3 Documenting Sustainable Building Cost Impacts spreadsheet - one of the tabs attempts to elicit a similar figure. But in the end, I wouldn't invest a whole lot of time or energy in this number. Hope that helps.