I would agree that for a small item of simple design the price of one good print is cheaper than the postage to have ShapeWays print and mail it.
But there are a couple of fallacies about how cheap a home printer is:
The home printer can be persnickety in getting ONE good print... it might take several tries before you get that ONE good print, so the cost of the failed prints must be included in the overall cost of that ONE good print.
Part of the failed print cost is because the machine is not setup properly (level, in a controlled temperature/humidity room, clean, etc.) Such that it takes several tries to get that one cheap print. You really need to include the cost of the failed prints in the price of the ONE good one. ShapeWays absorbs all that cost (well, we all pay for it in the high price of the prints we do get!)
But another cost that is often ignored is the antsy-ness of the "lazy" owner of a home printer... we might design the part and fail to verify the design while still in the computer... why not print it now and then discover the errors in the print? I know people that fall into that category (me, included, not in 3-D prints, but in cutting 2-D vinyl decals!)
Well, the cost of the failed print(s) to discover what could have been discovered by just stopping to really look at the design to see if it is as good as it must be to get a good print, needs to be included in the onesy/twosy printing that the home owned printer usually does. In commercial printing, that cost is minimal compared to the printing of thousands of the same item, but is a major portion of the expense of a single printed item. Not to mention the wear and tear on the machine making things that won't be good enough to keep. And if the design has a major flaw, it can break the printer such that the cost of that ONE good part needs to include the cost of repair or a new printer altogether.
When I print via ShapeWays, I spend a lot more time inspecting my design rather than pay such high prices for a print AND POSTAGE to get a failed design delivered to me.
And that home printer limits you to one or two materials, whereas ShapeWays has dozens of materials to choose from.
There is something to be said about the immediacy of seeing your design come into physical existence right before your eyes. And you might perfect a design on the home printer and then upload it to ShapeWays to have it printed in 18K Gold or some other material that you cannot possibly do at home.