> Here in the US I've heard families earning from $30k to $200k, working jobs from manual labor to white collar, call themselves "middle class."
Sounds perfectly reasonable to me. Where I'm from, the most important marker for middle class is that you value education very highly, and that you don't value physical labour above any kind of work.
Obviously, the people in your example at the low end of the pay scale would most likely be lower middle class, and those at the upper would be upper middle class. The major distinctions between those would be how to perform social interactions and the size of their social networks, but they all agree that life's goal is to send your kids through college.
(As opposed to being working class which means you do not value education highly, and you value physical labour ("real work!") higher than any other kind of work.)