According to Pronterface, it takes 8.1 metres of 3mm filament to print Wades Geared NEMA 17 Extruder. If you weighed one metre of filament, you would get a valued for grams/metre.

grams/metre divided by 1000 should give you metres/Kg.

[metres/Kg * (0.003/2)^2 * pi] will give you cubic metres per Kg

Price of a roll divided by metres per Kg will give you a price per metre.

Pronterface will give you the number of metres of filament required. You have to do these calculations for each print job. You can reduce the amount of material required, and speed up the printing if you nest the components into the smallest print area possible.

Old Man Emu