writes "Just how many books a year would you need to read before the cost of Amazon's Kindle is justified? The answer is not so cut-and-dried. If you're a college student and all of your texts were available on Kindle (possible but unlikely), you could recover the cost of the reader in a semester and a half. For consumers to break even with Kindle's cost in that time, they would have to be in the habit of buying and reading four new hardback books per month — if the convenience factor wasn't part of the equation. At two books per month, breakeven would be in three years."
Here is the spreadsheet
if you want to play with the numbers.