I've checked this for you and it appears that the price for the upgrade is half the main price of the version 13.
This is what it says:
This upgrade will work for Dragon NaturallySpeaking Premium 11, 11.5, 12 and 12.5. The serial number from one of these programs must be available.
Access the following website in order to initiate the upgrade procedure. As stated above, it's mandatory to have the serial number for the previous version of Dragon NaturallySpeaking.