My experience with embedded hardware type startups would lead me to think you would pretty much double your FCC registration process/expenses, add about 50% overhead to your QA budget to basically test everything twice, add about 20% to your hardware layout budget (unless the two chips are highly pin and form-factor compatible), and a few other incidentals. On top of the other extra basic engineering code to "get it all working".

Then of course you also have to WANT to have the unit on the carrier that you added the CDMA chipset for in the first place.

The harder part to gamble on in a situation like this is: how many people will switch carriers to get a truly hot device? If you get it right (as Apple has appeared to), a large majority of people will pick the phone over the carrier. I'm sure they would have sold more iPhones to date than the current number if they had a CDMA version, but I'm not convinced it would have been a truly significant number overall.

Anyway, that's my rough experience with hardware design. What's your perspective on the added costs?