I accounted for both as I like to add details for the author and readers.
If we use the current price, which one, the with spool or refill?
In the U.K. the current price is £18 for a refill or £21 for with a spool. As stocks are usually very fluctuating in availability across the board, today I could have bought a refill in white and print tomorrow with it only available with a spool, a deviation of £4.
So, for accuracy sack, I could be £18 or £21 if we discount all the potential discounts available which is when most order them and no accounting for gift cards which people do whenever they can.
Yet your question had this as a requirement.
You can’t have both, you can be accurate while not being accurate.
I have been lucky enough to earn sufficient points to purchase filament at no cost. This makes the accuracy exactly 100% out.
Granted I could opt out, but then for me, the energy cost is the thing I want as it is the only cost of printing when using BL filaments.
My point is, you can’t require accuracy and ignore all the factors.
The most accurate answer for you would be to manually type and average purchase price you manually add to a spreadsheet each time you purchase.
Just a list of each filament purchased with a quantity and price per unit in each row and a rolling average of all rows counted divided by the sum of those row totals.
You enter that average cost in the box as that is your most accurate estimate.
Not ideal, unless you love spreadsheets, but the most accurate way of getting filament.