I know you can ask Seeed Studio for a quote to make a product based on your design, they take care of my USnooBie kit (see my signature), and I know Dangerous Prototypes gets all of their products made with help from Seeed Studio. You don't have to sell your stuff on their site, you can ask them to make a bunch and ship them directly to you.
The Arduino is not cheap. Arduino shields are not cheap. You are much better off just creating a ATmega based design of your own.