To break it down a little
Cost Per Book Read
Devices: $600
Books: $1480
Total Books Read: 231
Cost Per Book: $9 or (600+1480)/231
Note: This is a lifetime cost so far. Typically when I buy a new device, I sell an old one, so this keeps the cost manageable. $50 each on two Palm PDAs (one to replace one that broke), $200 on misc stuff and losses from devices I sold and did not get the full amount I paid and $300 for the Kindle.
Cost Per Purchased Book
Total Spent on Books: $1480
Total Books Purchased: 479
Cost Per Book: $3.08
Note: Again, these are lifetime costs. It's very easy to track with a simple spreadsheet. I used to play the Fictionwise promo game hard, so often I would buy a book and get a rebate, then spend the rebate on several others. I don't count free downloads from the Sony store, but I do count the Fictionwise freebies I got with micropay and stuff like that.