The price of bulk 4GB flash memory chips is around $150.00 right now. Apple of course worked out a volume discount with Samsung on their flash memory for the iPod Nano, so they are paying around $75, IIRC. Even so, I would imagine that higher capacity flash memory chips would not only be very much more expensive, but also very likely they would be incompatible with Nano hardware. I haven't seen any pictures detailing the so-called daughter card containing the flash chip, everything I have seen has the chips soldered. I would personally be interested if you could forward that resource to me that shows the daughter card hardware.
Personally, I really suggest waiting on this one, seeing as the Nano was only just released about a week ago.