I don't have any experience of buying traditional banners, although I sometimes buy text links. What is a good price to bid in Google Adwords for CPM? $1 ? $10 ? Is this always going to be more expensive than PPC on Adwords for Content? Also any feedback from users of this site targeting about the conversion rates on, say, a CPA offer?
CPM ads compete with CPC for placement. The prices needed to get impressions will vary from site to site.