Maximizing Ad Placement-Bid price for those keywords

As an advertiser, it’s important that you pick the most effective
 keywords for your PPC ads. Pick keywords that no one is
 interested in, and your ad won’t appear anywhere; pick more
 popular keywords, and your ad will appear more
often—and, hopefully, generate more traffic back to your
site and more sales.

The problem, then, is that the best keywords are also the
 most popular ones.That is, lots of advertisers will be
 interested in the same keywords. Because ad
networks set their rates by having advertisers bid
 on keywords, the most popular keywords will cost
you more than less-popular ones will.

This competition for keywords among advertisers becomes,
 essentially, an online auction. Those advertisers who bid the
 highest amounts for keywords will “win” more and better
 ad placements. If you don’t bid high enough on a popular
 keyword, your ad simply won’t appear as often. In fact,
 if you bid way too low, your ad may not appear at all.
The temptation, then, is to bid high on the most popular
 keywords in an attempt to get more ads displayed and
 drive more traffic to your website. Be careful what you wish
 for, however, as this approach can result in very high
advertising bills.