[zhemao]

The additional marginal cost/unit is pretty low, but the non-recurring engineering cost is pretty high, even without having to deal with suppliers and the FCC. Once you've chosen a wi-fi module, where do you put it on the board? How do you make sure the radio signals don't interfere with the rest of the circuit and vice-versa? It's much easier to just get a USB wi-fi adapter and install the Linux drivers for it.