That said, not all shared hosting is bad. There are some decent web hosts in this space, including Hostgator and BlueHost. They can't handle thousands of dollars worth of traffic, but they should be ok for medium sized budgets. The thing about shared hosting, is that you are sharing your server with a bunch of other people. So even if you aren't using all your server's resources, someone else on your server could be. They could slow down your sites, or even worse, cause them to go down.
There's nothing worse then being in the middle of a media buy for your campaign, and your fricking server craps out on you. Paying for clicks that lead to no where is probably the worst feeling in the world. Check out this post that shows what can happening when your web host screws up.

I know most marketers are reluctant to invest in dedicated hosting, but at least look into investing in a decent VPS hosting provider. A company like WiredTree, BurstNet, ServInt are reliable companies and you can get VPS hosting from them for around $50 a month. This should be enough to handle traffic from your pay per clicks ads or medium sized media buy.
Remember, before sending a surge of traffic to your chosen web host, test it out first with a smaller campaign. See that your chosen host can handle the traffic first before scaling up. It's better to lose $100 on a small campaign, then a few grand on a big one.
So keep this in mind, before you start scaling your CPA campaigns up. Sometimes incentive marketers don't think about things like this until it bites them in the ass later. So don't say I didn't warn you.