The Next generation of keys, with pressure from insurance companies, and laws from the government came the Transponder Key. It was designed to lower car thefts, or increase the cost of keys, you chose.
In simple terms there is a "Chip" located inside of each key that transmits a unique signal to your cars computer allowing your car to start. Every manufacturer decided to put there own take on this, and instantly keys all of a sudden became very expensive. It seems like they change them every couple of years to make them harder to replace. These keys are expensive to replace, some more than others, and you should really think about have more than just one key.