Being "middle class" is a mindset, not a category of wealth. It's something of an anachronism from when there were two distinct classes: nobility/clergy, and the peasants. The middle class, the bourgeoisie (because they lived it towns, bourgs) didn't fit into either existing class. One can be wealthy or poor and still be middle class. Social conformity, infinitely expandable material demand, and emphasis on external, impersonal values are the hallmarks of this mindset.