It takes more effort to manufacture, market, and sell most products then it does to do the design. Keep in mind that the non design efforts are ongoing.
For a hint of what that is like check out the blog kept by the guy behind the Saleae Logic analyzer. https://blog.saleae.com/
You don't take an Arduino into small scale sales, it's not a chip it's a development system - you simply take the cheap AVR it's based on (or a cheap PIC) and build the device round that.
We're currently waiting for 100 sample PCB's for the PIC based device I'm involved in designing and programming - when it goes in to production it will be manufactured by a third party - the guy in charge of it all has experience in small/medium scale manufacturing in that way.
Exactly as Nigel put it really, you build round your cheap PIC or AVR, i myself have a product in development that uses a 44pin TQFP PIC, they cost £1.79 each. Thats a tiny cost when you realise the potential the chip has.