I'm looking for USB to serial FTDI cables. However, the cheapest I can find is $20 US each, which is way too expensive, given my price target for the entire product is $70. I'm hoping they can be found for $5 each somewhere, but even FTDI themselves has them for £16 (about $25) each. Anyone know of any other options?
Nokia data cables are USB to serial.
You might also want to look at non-FTDI serial to USB chips, like the Prolific 2303 (PL2303) which is in this Nokia cable.
I've found that the PL2303 is far more reliable than FTDI. They both do the same job though.
You can find a full adapter here for under $4: http://www.dealextreme.com/details.dx/sku.5859
If you do a Google Product Search for PL2303, you will find others.