First of all, pirating games is not why pc games are cheaper...console games can be hack/cracked just as fast and easy as PC games....it's just that modding or cracking the console itself is something that also has to be done in order to play the games, while you don't need to hack the pc to play the hacked pc games.
Secondly, it's easier making a game for a system that is the same in millions of households than it is to make one where the machine it will be played on is different in every household. That's the main reason PC games are cheaper...but along with the piracy issue and the fact that most gamers are all different ages which means there are less PC gamers then there are console gamers, it makes sense to cater to the bigger group....while at the same time, you do just enough to throw the smaller group a bone and keep them slightly interested as well....PC gamers are that group when it comes to games that are primarily catered for consoles....console gamers are that group when it comes to games that are primarily based around online game (such as MMO's).