In a fair world we would allocate funds for medical research based on number of people suffering from illness and not their purchasing power -- but then the world isn't truly fair to everyone.
Imagine this were happening inside a country -- a large percentage of population with income below 60K suffered from an particular disease A and a very small chunk of people earning more than 120K suffered from a rare disease B. And everyone including the government decides to fund research on disease B -- won't that be unfair and unjust ??
Also with people shunning vaccines and emerging varied forms of resistance, Disease of the so called "Poor Developing Countries" might come back to haunt the "Rich"