Criteria for choosing which regions to use on a multi-region setup for improved reliability

There was a recent blog post on this: "What to Consider when Selecting a Region for your Workloads" https://aws.amazon.com/blogs/architecture/what-to-consider-when-selecting-a-region-for-your-workloads/ Hope this will help you identify the criteria that you could use.

From the blog post above, you will likely mainly consider latency (some regions such as Stockholm are cheap to use but would significantly impact your US/SA userbase's experience) and cost as you mentioned.

Cost though is something you can tune and optimise by carefully consider what you need to replicate (this will be based on your risk modelling). For example, if you go for active/active and plan to do old-fashioned backups, you can keep them in the cheapest region. Traffic flows can be optimised to avoid going too much "cross border" (ie make sure once an user's request enters into a region, it stays there).

I know you haven't directly requested it, but if you're planning things now, I suggest to do it with this in mind: https://aws.amazon.com/builders-library/static-stability-using-availability-zones/ - most of Amazon/AWS are designed based on this principle and generally limiting your dependency on control planes or changes in general during a failure will make everything better.