One of the biggest costs is obtaining the data. I am no longer involved but we employed a freelance IEMA consultant as lead assessor to guide us but we obtained the data ourselves initially to keep costs down. This however did help us in the long run. I am no energy consultant but 3 offices should not be too difficult to do. We had all sorts of buildings, machinery and users. We used a combination of data loggers, meter readings, HAV settings and user logs. I was on the side lines of this project bit I remember facing the same issues as you, wildly varying and some silly cost.