About 5 cents. A 50 W desktop dryer running a six-hour PLA cycle uses 0.3 kWh, which at the US average rate is barely measurable.
A longer, hotter cycle is barely worse: 12 hours at 80 W for nylon is 0.96 kWh — 17 cents.
The comparison that matters
A single PA6 spool costs $35 to $70. Printing a wet one produces steam-pocked, weak, fuzzy parts and usually ends with the spool going in the bin or spending a day in the dryer anyway.
So the real arithmetic is 17 cents against the risk of a $50 spool and a 9-hour print. There is no version of this where drying is the expensive choice.
The cost that is not the electricity
Leaving a dryer on permanently as a dry box is a different proposition. A 50 W unit running continuously for a year uses 438 kWh and costs $76.65 at the US rate — more than a spool of PA6, and more than the whole year's electricity for a printer running eight hours a day.
If you need continuous storage rather than a pre-print cycle, sealed containers with desiccant cost nothing to run and work at least as well for materials that are already dry. Use the heated unit to restore a spool, and a sealed box to keep it.
Which filaments actually need it
The data layer on this site marks drying as required for most materials and optional for PLA. In practice:
- Nylons and PVA absorb fastest and need 8 to 12 hours at 80 °C, then need to stay dry during the print
- PETG, ABS, ASA and the carbon-filled grades want 4 to 6 hours at 65 °C after any long shelf time
- PLA tolerates humidity better than any of them, but a spool left open for months does print visibly rougher, and 4 to 6 hours at 45 °C fixes it
The one real risk
Drying temperature above the material's glass transition softens the filament on the reel and welds the coils together, which ruins the spool as thoroughly as moisture would. The schedules above are below that point for each material; a kitchen oven set by eye is not.
Cost your own dryer's cycle by entering its wattage and hours in the print cost calculator, and see per-material schedules in the drying guide.