Routine questions for the makers of desktop 3-D printers: "what do you do with this thing?" and "why would you want one?" A study from Michigan Technical University answered that with some math: the researchers found that you could recoup the up-front cost of a 3-D printer in less than a year.
The researchers used a RepRap printer and printed about 20 household items, from shower rings to iPhone cases, and found that the total cost of materials was about $18. (It's a bit of a change from this cool stuff.) The "lowest retail cost we could find for the same items online was $312 and the highest was $1,943," said Joshua Pierce, one of the researchers. The study projected that a 3-D printer could could save consumers between $300 and $2,000 a year.
They're assuming that you're using one of the cheapest RepRap printers, rather than a RepRap-based printer like the MakerBot that can cost a few thousand dollars, and also assuming that you have a lot of need for plastic items, and also assuming that you like the quality of items printed with this kind of printer. With a 100-micron resolution, you're getting a rougher sort of product than you'd buy from Amazon; you can see the layers, for example. But the benefit is that you can modify the items to your preference, with different colors or patterns or sizes.
"The unavoidable conclusion from this study is that the RepRap [3D printers] is an economically attractive investment for the average U.S. household already," concludes the paper, published in Mechatronics.