Bandwidth wise I do use substatially more, but really it is number of visitors, and pageviews that make a difference. Our conueries are 1.185MCn estimate for this month. Way under the allowed 15MCn.
Directly related to your point, I scrolled backwards through the Dreamhost admin panel (you have to admit that the admin panel is very nice, even if slow) to a date before I implemented cgi, and my conueries doubled around that time. That is for one page, that gets about 500 views per day, and that is the ONLY page on my entire site that is pcgi, although all other pages do use php and mySQL.
I hope this helps,