When you create a new Site Targeting Campaign, one of the options is to choose from CPC and CPM
What I would suggest is that alongside the site-targetting campaign (whether CPI or CPC based, more on that later) you spend time customising your adverts for that specific website. Be as precise as you can and drive up your CTR. With a better CTR you will be able to justify a higher CPI basis, and on a CPC basis you will get that additional exposure without having to increase your bid.
Too many people think of Adwords as a shop. You go in, you buy, the more you spend the more you get. That's the wrong way to look at it. The limited resource is not your money, it is the website real estate. Practically every site in the content network is a mini-battleground between advertisers who, knowingly or unknowingly, compete to bring the best combined CTR*CPC*QS and get the pageviews. Think of your aim as to increase CTR*CPC*QS first and foremost.