As you are aware of different types of advertising model like CPA, CPI etc. but here it matters a lot which countries are you targetting. There will be a great impact on ROI based on targetting countries. Lets first understand what exactly mean by different tiers of countries. The grouping of countries into different tiers is based on GDP and purchasing power. Depending on this , there are generally three types tier, Kindly refer list below:
Tier 1 – A set of countries that every Marketers/Advertisers are willing to work with. They are rich countries and the most competitive GEOs. The people here have good per capita income and better purchasing power. These countries are very competitive, High payouts, Very expensive traffic.
Tier 2 – Lower per capita income as compared to tier 1 and these countries are quite competitive, comparatively lesser payout, less expensive traffic.
Tier 3 – Developing countries and consumers with a low purchasing power. These countries are Extremely low competition, very less payouts, Very cheap traffic.
Depending on advertisement goals as well as business model of the product that is advertised the advertiser should decide about the targeting. Income from ads / eCPMs are likely higher in tier I countries.