I came across this post on Cnet from April of 2009 where they were talking about the Kyoto Box being a $5 solar stove. The rough prototype made me think that this is really something that could be done at home. Meanwhile, Kyoto has gone all chick with their products, and have a really smart looking website and some cool products ranging from 5 to 300 Euro all taking advantage of free energy sources.
But while 15 Euro is really not overpaying for a nicely produced Box, the Nature Craft Project has done a nice writeup for how to make a really simple solar cooker using just a box and some aluminum foil. If you get a chance to try either please let me know how it turns out!