In the US, where I am at, Whispernet itself is "free" - which means to me that I paid for the access in the K2 cost itself. So browsing to Wikipedia or Amazon, or any other web page, has no inherent cost. However, Amazon does charge for the service of converting and sending a document (which can be an ebook that you got outside of Amazon)through Whispernet. To me the charge is minimal - it is based on the file size. Finally, you can avoid that charge by sending it to
yourusernamehere@free.kindle.com rather than
yourusernamehere@kindle.com. The free.kindle.com address has them convert it and then queue it for download to your K2 through your PC via Internet/USB rather than Whispernet.
One more thing - Calibre is a free e-book library management program and it does a great job of converting formats and uploading files to the Kindle. It also has the ability to pull newsfeeds and create 'newspapers' for you to read on your K2. I highly recommend you take a look at this one.
I hope this helps, welcome to the Kindle world.