But the SECOND most common question is, "How much did it cost to print that?". I usually say "negligible" - since a 1 lb spool of 3D Printer PLA filament costs between $20 and $30 - but I figured it was time to calculate a better answer.
I buy PLA of both 3mm diameter (for my Lulzbot TAZ4) and 1.75mm diameter (for my Polar3D). I'll stick to just 3mm here for simplicity.
1kg Spool of 3mm PLA ranges widely - from $23 (Amazon) to $44 (Ultimachines).
|CURA slicing app gives length estimate|
That means the cost of a print is approximately $0.21 - $0.40 per meter. Of course, that does not include electricity, loss due to bad prints and testing models before final prints. It also doesn't include the cost of things you may smash when frustrated over failed prints (no, I've never done that).
If you're lucky enough to use a slicing app which gives approximations on the length of PLA required for your print, you can make this estimate. Cura does this - as you can see in the picture included here - where it gives the length required and the time estimate to print (using all the parameters for the given print job).
Google Docs Logo/Icon (40mm): 0.56 meters = $0.12 - $0.22 per object
Filament Spool model: 4.19 meters = $0.88 - $1.67 per object
Raspberry Pi Box with top: 6.33 meters = $1.30 - $2.53 per object
Makerbot also has a great post (albeit from 2012) on this topic which estimates that you can print 392 chess pieces with a 1Kg spool of filament.
Answer = cheap. So, negligible is a good answer when you're printing just a couple of keychain trinkets, and if you are printing larger objects or large quantities for a professional job perhaps, you can possibly use the 20-40 cents per meter of 3mm filament as an estimate.