On the Coinotron, multi-currency pool website, there is a metric posted at the top-right of each page that displays how "profitable" it is to mine a specific currency.
How does Coinotron derive this value?
It divides the block rewards given multiplied by the value of the currency in USD by the difficulty to generate a this same reward on each cryptocurrency.
They compare how many BTC you get per day if you mine BTC, vs how many BTC you get per day if you mine LTC, TC, etc.
The percentage is how much extra you get, but it makes a lot of assumptions and generalizations.