High-volume keywords and targets are the most competitive, which drives up bid prices. We recommend bidding the Smart Bid if you want maximum volume, but you can get traffic with lower bids, too. Your Bid Graph can provide some insight to the competition, and you can set Proxy Bids on Pops to cut costs while bidding competitively.
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It's also a good idea to make sure your targets aren't competing for the same traffic, which can cause your Smart Bids to increase every time they match for the same impression.