A 3D printer can pay for itself within a period of two months to two years, according to a paper published by some rather serious people in Mechatronics.
The paper is titled Life-cycle economic analysis of distributed manufacturing with open-source 3-D printers and shows that using a 3D printer to make only 20 products a year results in avoided purchase cost savings ranging from about US$300- to US$2000-a-year. Because the printer can also make it’s own spare parts “the unavoidable conclusion from this study is that the RepRap is an economically attractive investment for the average US household already”.
I can see this working but it does depend very much on the sort of product you buy and are willing to replace with a 3D-printed version. The full article sits behind a pay-wall that I’m not willing to breach so I can only go on the abstract. But in the abstract the example given is a spoon stand. That’s actually quite a good thing to 3D-print. But it’s an even better thing to not have in the first place. I fear that the most easily replaceable things in most household are also the most useless and so this analysis will only work if you take the necessity of the underlying objects as a given. But if you assume the objects are going to end up in the consumer’s hands one way or another, then I can definitely see that this makes perfect economic sense even today.
So, if you’ll excuse me now, I’m off to persuade my family that we need spoon holders so that my 3D printer can pay for itself.