Erick Schonfeld of TechCrunch says, We know how many Kindles Amazon has sold: 240,000:
Doing a little back of the envelope math, that brings total sales of the device so far to between $86 million and $96 million (the price of the device was reduced to $360 from $400 last May). Then add the amounts spent on digital books, newspapers, and blogs purchased to read on the device, and you get a business that has easily brought in above $100 million so far. (Each $25 worth of digital reading material purchased per Kindle, add $6 million in total revenues).
[...]
Scott Devitt, an analyst at Stifel, Nicolaus & Co., predicts that Amazon is on track to sell 500,000 to 750,000 more Kindles over the next four quarters (including this one). He estimates that Kindle owners will buy an additional $120 to $150 worth of books and other content for each device, bringing the total revenues over that time period to somewhere between $225 million and $355 million. Based on that, he values the Kindle as a $1 billion business for Amazon.