It's 20% of the price of the equivalent full license at the time of renewal. Here you have the current prices for support and updates licenses.
Example: A license for 10 users was bought on Jul 25th, 2013 for $149. If on Jul 25th, 2014 the price of a 10 users license is $180, the price of support and upgrades license for 10 users will be $36.
