Depends almost entirely on one thing: whether the thing you want is mass-produced.
A 100 g part costs $5.15 to print in materials, power and machine time. Add twenty hands-on minutes at $25 an hour and it costs $13.49. That is the real figure to compare against a shelf price, and it makes the answer obvious in most cases.
Where printing loses, and loses badly
Injection moulding amortises a steel tool over hundreds of thousands of parts, so the marginal cost of the millionth storage box is pennies. Nothing on a desktop competes with that.
Anything sold in a supermarket, anything with a moulded-in living hinge, anything that had to pass a safety standard, anything requiring a smooth cosmetic finish — buy it. A printed version will cost more, take longer and be worse.
Where printing wins outright
- Parts that are not for sale. A discontinued appliance knob, a bracket for a specific shelf, a jig that fits one machine. The market price is infinite, so any print cost wins.
- Parts sold at a specialist markup. Camera plates, drone mounts, RC spares and lab fixtures are low-volume mouldings priced accordingly. A $1.06 print against a $30 catalogue part is not a close call.
- Anything that needs to be tried three times. Iteration is what a printer is genuinely for, and each attempt costs the price of the plastic rather than a shipping cycle.
- One-offs where waiting is the real cost. A part in your hand this afternoon versus a part in three days is not primarily an economic comparison.
The trap in both directions
People who have just bought a printer count only filament and conclude that everything is cheap to print. People who have costed it properly count labour and conclude that nothing is.
Both are wrong for the same reason: they are pricing the part rather than the situation. The printer's value is not in undercutting shelf prices; it is in making things whose shelf price does not exist.
A quick test
Ask what the same object would cost delivered, then ask whether you would be willing to do twenty minutes of setup and cleanup for the difference. If the answer is no, buy it — and the printer has still saved you money by keeping you from printing it.
Price your own candidate with your own labour rate in the print cost calculator.