There is no list you can look up — it depends on your app. Here is how to find yours in a fortnight, and the trap at the end of it.
There is no list of cheap countries you can look up, and anyone selling you one is selling somebody else's app. A storefront is cheap for you when three things line up: few people bidding on the words your app answers, enough people searching them, and users there who behave the way your app needs them to. Change the app and the list changes.
What is true for everybody is the starting point. The United States is the most expensive storefront on the App Store and the one almost every developer runs first, usually because it is the default and occasionally because it is the only one they thought of. A tap that costs two dollars in the US can cost twenty cents somewhere else — and at a budget of five dollars a day, that is the difference between two taps and twenty-five.
The trap at the end of this road is real, so take it first. It is easy to find a storefront where installs cost a tenth of what they cost at home, and easy to discover a month later that none of those users ever paid for anything. A cheap install is worth having only if the person installing eventually does what your app is for.
So the number to judge a country on is not cost per install. It is cost per install measured against what a user from that country is worth to you — which you can only know after they have had time to subscribe, renew or churn. Until then, treat a cheap market as promising rather than proven.
The only way to know what a storefront charges you is to buy something in it. That sounds expensive and need not be: you are buying an answer, not a campaign, and the answer costs the same whether you like it or not.
Testing one country at a time takes a fortnight per country. Ten countries in one campaign take the same fortnight and Apple reports each of them separately, so the cost of the answer is the same and you get ten of them. Keep the group coherent — small markets with a shared language, say — so the keywords can be written once for all of them.
A Maximize Conversions campaign with nothing but Search Match turned on serves almost nothing. Measured on a live account, it delivered about one per cent of the impressions the same campaign got once it had actual keywords in it. Write the words people would search in each storefront's own language, not the English ones with a country code attached.
Apple reports impression share in whole Sunday-to-Saturday weeks. Anything shorter than a complete week is not a reading, and a single week of a new campaign is mostly Apple working out who to show it to. Two weeks is the first point at which the numbers describe the market rather than the learning.
Not taps, not impressions. A country that bought a thousand impressions and no installs is not cheap; it is irrelevant. Rank the storefronts by what an install cost, ignore any with fewer than a handful of installs — three installs is not a rate — and the winner is the one worth a real campaign.
The test campaign is a measuring instrument. Once it has answered, it has no further job: keep it running and it goes on spending to re-learn something you already know. Build an exact-match campaign in the winning storefront, with the keywords that actually converted, and turn the test off.
China needs separate approval from Apple before you can advertise there, so it cannot be part of a first sweep however promising it looks.
The United States is worth entering eventually even though it is the most expensive, because it is also the largest. The mistake is entering it first, with a small budget, and concluding from the result that Apple Search Ads does not work.
A fortnight at a small daily budget across a group of storefronts. If that sounds like money spent on nothing, compare it with the alternative, which is a year of running the US because it was the default and never finding out that your app sells three times better in Brazil.