CPM (Cost per mille)- Premium inventory, Premium Publisher, Premium Price
CPM means Cost per mille ("thousand" in Latin) and is the pricing method depending on 1000 expressions.
Almost all publishers would rather bill author on impressions because it's an inventory based product instead of a performance based product. In other word, publishers risk hardly anything on ad performance. With a CPM system, they get paid for every impression. For the largest and best publishers, here is the pricing standard plus terms of price tag, CPM priced media is normally at the top in the chain.
CPC (Cost per click)-realm with the small Publisher as well as DIRECT response marketer
CPC is short for cost per click. It is a performance based metric. This means the publisher only gets their pay cheque when a user clicks an ad. This hapens regardless of how many impressions they serve trying to find the click. As you can guess, this costs is much favorable to marketers, but can be difficult otherwise impossible to negotiate with any publisher using a premium brand name and name, especially with all the ad networks and ad exchanges springing up and buying unsold inventory and pay by CPM. Publishers don't like CPC pricing because it's difficult to plan inventory demand around a moving target like click through rate on an ad they have never seen or tasted before.
Different techniques with unique opportunities
Two campaigns sticking with the same CPC rate may need different a higher level impressions for the publisher to bill entirely and this uncertainty may very well be as a very high opportunity cost. It is only after exhausting remarkable ability to sell by CPM that this publishers entertain CPC offers, through that point, marketers are at the bottom from the barrel in relation to inventory availability and quality. If you are brand fighting for your attention of key demographic, this simply will not help.
For smaller publisher without much brand, selling their inventory on a CPC basis is often the only option left. The CPC media customers are huge, multibillion Dollar market and a lot of people make money by the clicks. Google's AdSense strategy is the largest CPC clearing house that pulls publishers and marketers alike in huge numbers. CPC is low risk and marketers simply have to pay for performance to allow them to have some level of confidence in their return in investment.
CPA rarely an amount option rather something to optimize ROI
Cost per action is best deal for advertisers in relation to its risk given that they only pay for media resulting in sale or conversion against their campaign goals. At this level marketers can come up the highest they may be willing to spend, set your budget and forget about it. Similar to CPC pricing this is simply not a great deal for your publisher. Affiliate marketing programs run on CPA basis with publishers which are exclusively devoted to hawking their product with sites which can be more advertorial than anything else.