To many first time PPC advertisers this may seem counterintuitive, but for Google especially I recommend starting with a higher bid and then gradually bringing it down to your comfort level.
The reason for this is the huge impact of Quality Score in Google AdWords. A good quality score will bring your click cost down significantly, and a low quality score will drive costs through the roof. One of the primary factors in quality score is clickthrough rate (for the keyword, the ad, the account as a whole).
Starting off with a high bid will lead to high ad ranking. Combining that with compelling, relevant ad copy will likely lead to a high click through rate, which will in turn reflect well on your quality score. This will make the whole PPC experience much more enjoyable, and hopefully profitable!