That's a whole different question and I don't think nobody has a definitive answer. Market rate seems to be a "consensus" of how much does a developer add to a company. But paying less to a developer just because they live in a different country throws that out of the window completely.
There isn't a deterministic formula that you can code, but the market generally does this.
A good backend python developer working for a London company here can get 80-100k GBP, if company X wont pay this, then company Y will. The developer goes to company Y.
If the job can be done remotely, and everyone is happy, then who the hell cares that this developer lives in an area with low cost of living? He's fulfilling the contract (quality work in exchange for money) and where they are physically located is irrelevant in determining their compensation.
You are talking about ongoing market rate, not the value output. Sometimes even skilled developer can produce negative value because of accident or smth.
Why would I pay remote worker the same as for local developer, if I can find remote developers willing to work for much less?