I was thinking about this question today as someone used our work printer for some personal stuff.
As for me, I am printing little things that I would say make it worth it. I’ve printed lens adapters for my camera for example. That’s worth a good 14 to 30 bucks per print. My most favorite photo was with an adapted lens that came from a projector. I also printed IEMs and those things are worth it. Listening to music is second to none on those things. Plus I printed the same shell but for ear protection and again the fit is perfect and sure there’s post processing to get smooth surfaces but in the end it looks like a professional made it. So I think 3d printers are worth it.
I wouldn’t buy a printer to try and be profitable per say, I would buy one for the things you cant/don’t want to buy. If you can use some 3d modeling software, you can begin to solve problems for almost no money.
Stuff like a vape holder and extended cup holder for my partners car, or a couple little shelf brackets for our IKEA cubes, or replacement closet rod supports. It takes a few minutes in CAD, a couple hours or printing, and 15¢ of plastic, and saves a trip to the store. Making custom, exactly how you want stuff is really nice.
Knick nacks are fun too, but ultimately your house will fill with things you don’t particularly want or use.