Remember that the Mbed board is a subsidized development system, the price you pay is not cost price.
I don't know what you mean. Who's paying at cost price anyway?
People buying Arduino are not paying the cost price either.
Arduino uno dev board - $30 bucks or so, ATMEGA328 @ Mouser $3.00 each ($1.90 for 25pcs)
mBed freedom dev board = $12.95, LPC chip @ Mouser $8.50 each ($7.65 for 25pcs)
From the pricing above, it looks like mBed is more likely at cost price, while Arduino is way overpriced.
(granted there are other supporting chips around, but you can see the big difference above).
of course, just buying the ATMEGA chip will be the cheapest solution and just breadboarding it or making your own board.