This is determined by the 'difficulty' of the coin, which fluctuates depending on the rate at which people mine it. The value of the coin is somewhat correlated with difficulty - coins that are harder to mine have a better value, which is why coins that pay out lots of rewards are worth the least upon starting. This is a basic principle and there lots of other factors that influence the coin's worth too, like the release, community reception etc. Have a gander at http://www.coinchoose.com/ and note the patterns between the number of blocks, coin reward, difficult, and BTC worth