For a start, the Vita's most expensive components (the OLED and touchscreens) are in massive demand for phones, tablets and so on. That pushes the cost up, not down (as makers vie to buy limited stocks) and as OLED is still a pretty new technology, it is sold at a premium. It won't match traditional LCD prices until well into next year, when prices might start to come down.
Pic from the Vita teardown by iFixit
Finally, the Vita hasn't sold as many units as it would have hoped, so Sony will have plenty of boxes and component stocks that it brought at the higher prices. It can't save money until it gets rid of the first-generation models, something that may take time (probably the reason why there are so many different bundles being produced).
Therefore, don't expect a cheaper PS Vita until well into 2013, even if sales rocket over the next few months as the game line-up improves. Sony is supposed to make a decent profit on each Vita sold, but can't afford to cut that due to the company's wider struggles. So, when the cut does come, don't expect it to be massive.