I hope it won't take to long until there will be pre-assembled ones, too. For an open source design, all you need is someone who invests some money to get the board assembled in China and then sells it in a web store.
Here is an example, to show how affordable assembling low quantities of boards in China has become. For one of our designs, buying all the parts online in the US or Switzerland, was about $150. If you buy about a dozen of them in China, they are about the same price point. But then, they are already assembled according to our design! Once you buy around one hundred, you save considerable money. In our case, the offer we got is about $100 per board (33% savings!).
The lower the transaction cost, the better. If you think soldering has minimal transaction cost, compare the price of a soldering station with a Raspberry Pi.