It was only £300...
...if you consider your own time utterly worthless. Which you clearly do.
For Mr. J. Public, who is famously unfamiliar with IT components and lacking in the skills to build small PCs from scratch, this is actually a pretty good deal. Sure, you can buy something cheaper from the competition, but Apple clearly don't appear to be suffering from being seen as the Rolls-Royce of the IT industry.
As another poster pointed out, this new Mac mini is a major work of miniaturisation on Apple's part—have a look at some of the strip-down pieces on the older Mac mini models if you don't believe me—so good luck building a £300 clone of using off-the-shelf components and cases. This new model is *tiny*, and that's worth money in itself.
Apparently, many members of the general public really do appreciate good user experience design, and are willing to pay for it. Who knew?