Why the number is bigger than grams × price
Purge towers, skirts, the tail you cannot print with, the first layers you scrape off — none of it ends up in the part, all of it leaves the spool. The calculator adds a waste percentage on top of the part weight for exactly this reason.
The one case where it must not: a weight taken from a sliced file already includes supports and purge, because the slicer counted what the nozzle extrudes. Adding waste again would charge you twice for the same plastic.
What filament alone does not tell you
Plastic is rarely more than half of what a print costs. Electricity, the wear of the machine, and failed prints make up the rest, and none of them are visible on the spool. The breakdown above shows all of them side by side.