How much does this 3D print cost?
Part weight, spool price, print hours: the true cost including energy, printer wear and the share of failed prints.
Why it's not just filament
Filament is often the smallest item: energy matters too (a printer draws 100-150 W on average), plus machine wear spread over print hours and failed prints (5-15% is normal). Ignoring them means losing money, especially if you sell.
The selling price
The suggested markup (default 200%) covers your prep time, post-processing and margin: for commissioned work most makers use between 100% and 400% depending on complexity.
From the spool price to the part price
Filament is bought by the kilo and used by the gram: a one-kilo spool at 22 costs 2.2 cents a gram, so a 45-gram part is barely a euro of material. It's the number to keep in mind, because it shows immediately how little filament weighs on a long job: twelve hours of printing may use fifty grams of plastic and a great many more machine hours.
You don't have to guess the weight: the program that prepares the print states it, along with the expected time. Those are the two numbers to copy in here, and they're also the two that change most depending on how you set the infill and the wall thickness.
Not all time is the same
The printer's hours cost little (energy plus wear); your hours cost a great deal more, and they're the ones that get forgotten: preparing the file, sorting out the supports, taking the part off the bed, removing the stringing, sanding, gluing, packing. On a small object the handwork can exceed the cost of everything else put together.
Failed prints belong in the sum
One part in ten failing isn't bad luck, it's normal for anyone who prints a lot: a first layer that won't stick, filament running out halfway, warping off the bed. Anyone selling has to spread that cost over all the good prints, otherwise every failure eats its profit. And it's why a long print is worth less than it looks: the longer it runs, the more chance it has of failing three quarters of the way through.
Nearby tools
To set the price starting from the cost, with margin and markup kept apart, there is Margin and markup; to know how many units cover the fixed costs Break-even point. If you work to order, the rate you actually need is worked out by Freelance hourly rate, and the energy cost of an appliance left running by Electricity cost.