First, do an inventory of consumption.
Second, define your current practices: maintenance schedule, operating parameters (hours of operation/day), week end use etc.. Ideally, you would use an energy draw profile but just defining practices is usually good enough.
Third, determine your energy costs: $/kwh, or $/gallon of LP if used.
Fourth: determine % of full load capacity = Full load CFM/Required CFM. Req.CFM would be a normal consumption as found in step 1.
Fifth, Calculate cost. The formula I use is:
(%Full load X Full load BHP X .746(kw/BHP) X Hrs/shift X Days/week X Wks/yr X $/kwh)/ Motor eff..
This gives an annualized operating cost figure. From there you can add maintenance and other costs to develop totals.