As Mike said, for buying books the cost is built in price of the book. We pay an extra $2 for all books, even if you don't use Whispernet to get them on your Kindle (my pet peeve).
Using Whispernet to
- Get samples
- Download books you've bought
- Re-download books you've bought
- Browse the Amazon store
- Browse Wikipedia
are all free, aka paid for by the extra $2 you pay for the books.
Anything else may cost you $1/Mb. This is mainly browsing other web sites, and receiving converted items that you send to
your.name@kindle.com.
To avoid accidentally costing myself, I
- Leave wireless off, and
- Don't have any approved emails (set in Manage My Kindle)