Clicking the Publish button will pop up a dialog to confirm the transaction and set the data for you. The price you will pay is 0 ETH + the gas price
In the sample above the transaction included 913 bytes of data. The cumulative gas used was 104084. The actual cost was $0.56. The gas limit is right now around 3.14 million. There is a limit on the size of the pages which would be the size that hits the max gas limit. It would get more expensive to store larger web pages but for basic pages or text it is cheap to store forever.
Listed on the Dapps directory State of the Dapps at Ethercast.