Not to mention, we have no idea what the cost is on a Kindle. Netbooks use commodity components that are widely available from numerous manufacturers. The same is not true of e-ink displays. This is at least partly a factor in the perceived high cost of an ebook reader.
We also don't know what kind of arrangement Amazon has with Sprint. Amazon gives a few cents to Sprint when you purchase something using Whispernet. Maybe Sprint also gets a flat fee every time a Kindle is sold. We simply don't know.
There's also the issue of limited competition in the ebook market right now. There are only a few manufacturers of quality e-ink readers and on top of it all the Kindle is the only game in town if you want to buy ebooks from Amazon. This is relatively new technology and the price will go down. We've seen it before with the components that go into a $250 netbook. This is economics 101.