How do you cost a CE for Inflation before the actual costs are agreed.?
I assume you are asking how an inflation allowance is included within a CE quotation. This would relate to the forecast Defined Cost, as any incurred actual Defined Cost should be established.
It would depend upon when the work is planned to be undertaken and whether the associated costs are subject to unusual fluctuation, such as volatility in the price of certain materials.
If the effects are too uncertain to be forecast reasonably the PM could state assumptions, with the matter dealt with as a further CE if the assumptions are found to have been wrong.
Alternatively you could include an appropriate risk allowance.
If you are using Secondary Option X1, then this would change the approach, especially under Main Option C or D, as an inflation adjustment amount is calculated, based on the Price fior Work Done to Date, which is then added to the Prices (Target).
This calculation can be quite generic, however, and may not compensate for a specific item, so any ‘difference’ would not to be addressed specifically in a CE quotation.