I think you mean, how much does CPM convert better than CPC? Both pricing forms are usually used for completely different purposes. High level: CPM typically is used for branding campaigns while CPC is used for direct response. Identifying an ideal overall conversion rate depends on sooooo many variables I can't even begin to get into it however if your ROI model looks at PPI (profit per impression) then I would suggest checking out Brad Geddes Blog Post about this subject. (Do a search for his name and profit per impression). Its pretty detailed and gives you a clear understanding of reporting on this metric. Hope this helps!