I wonder what job you do stuarty? I am self-employed, definitely do not overcharge and do a bloody good job to boot. However, my overheads (which people probably don't even consider) are fairly substantial - insurance, electricity, heating bills, machinery, website hosting and design, phone bills, staff wages, expensive printing inks (I use various types and the price of some makes your eyes water), stock costs, advertising (huge amount is spent on this), bank charges, business rates etc, etc.
If I did some work for you you'd probably think, she's only got an old Roadster so why should I, with my 45K worth of cars, pay her a living wage?