A PPC campaign, when done well, should cost you less and less as time goes by. There is quite a finesse to it.
Some simple tips are to always have multiple ads running for different keywords groups and always have two different ads running for each group so that you can observe which ad performs best - then tweak the underperformer and watch for results. So you'll always be improving. And of course, with Google, the better an ad does, the better placement it gives you and you spend less per click.
Have you ever read a good book about PPC? There are some good one's out there.