Take a real job: 48 g of PLA at $22 a kilogram, 5.2 hours, a 120 W machine, 50 cents an hour of machine allowance. It fails at four fifths of the way through.
You threw away $3.01. Filament $0.84, electricity $0.09, machine time $2.08.
Note the shape of that. The plastic — the thing everyone fishes out of the bin to look at — is a little over a quarter of the loss. The occupied machine is two and a half times more.
The bigger cost is the one that is not in the list
That printer was busy for 4.16 hours and produced nothing. If it is a hobby machine and it would otherwise have been idle, the slot cost you nothing and $3.01 is the whole story.
If you print to order, the slot was the scarce thing. A machine that fails one print in five does not lose 20% of its material; it loses 20% of its capacity, and capacity is what you were selling.
How failures actually enter a price
Not as an addition. If a fraction f of attempts fail, you need 1/(1−f) attempts per finished part:
| Failure rate | Attempts per good part | What it does to the bill |
|---|---|---|
| 5% | 1.053 | adds about a twentieth |
| 20% | 1.25 | adds a quarter |
| 50% | 2 | doubles it |
A 20% failure rate is a 25% cost increase, not a 20% one. That gap is trivial at low rates and violent at high ones — at 50% the same job goes from $3.77 to $7.53 — which is why the honest response to a high scrap rate is to fix the process rather than to price around it.
Failures caught early cost less
The model above assumes the failure wastes a full print. Many do not — a first layer that will not stick is usually spotted in minutes, and costs almost nothing. If your typical failure is discovered at four fifths of the way through, the full-waste assumption is right; if you watch first layers and abandon early, you can honestly halve it.
Measure it rather than guess: count the prints you started and the prints you kept over a month. That single ratio is worth more than any published failure figure.
Put your own rate in the print cost calculator, or read the full treatment in the cost of a failed print.