If you can buy something mass produced that exactly fits your need then that's almost always going to be cheaper. What 3d printers are good for are one off or low numbers production runs of unusual stuff, personalised or customized to exactly what you want. Or designed from scratch to solve a unique problem. Of course they're fun to play with as well and you can print other less useful stuff once you've got one like board game pieces, decorative vases, well anything you can dream up really.
How I justified finally getting one was I needed some low profile trickle vent covers for our windows and just couldn't seem to find any. So I've designed and printed some and they are perfect, just what I wanted. Of course spending £1000 on a printer to get them is a bit over the top but it was a good excuse!
Andrew