Nand Flash for USB thumb drive

That seriously begs the question: why? Do you really believe there's anything left to be earned with such small sticks, when the things routinely available in stores right are a good deal larger than that, and have certainly seen enough competition among each other that there's unlikely to be much slack left in their prices?

You are right about the plain vanilla USB memory Sticks but there is room for special memorysticks where you copy data over a proprietary interface to the stick and then you can easily read it from a PC.

Even with at least a couple of "dual use" USB sticks, e.g. memory cards that can serve as both an SD or CF card, and as a USB stick, already on the market? And even if not: why would that 9 US$ price tag on the memory chip challenge a proprietary, limited-purpose production? If it's proprietary already, what could possibly keep them from charging 50 or 100 bucks for the thing?

Because those sticks will be sold in thousands or tens of thousands of units.

So what? The term "proprietary" still means that, pretty much by definition, there's no competition to worry about. So, again: what could possibly keep them from charging whatever they want for those things?

Company A, has an interface and ask company B for a quote for 10,000 units. If it is too expensive, Company A will ask company C...

Company D will choose between Company A and Company E which both delivers similar products.

If you can sell at high price, low cost will increase you profit margin.