Would love to hear your feedback on the below approach.
Start by clarifying that the question requires an estimation in USD.
1/ Global Demgraphic (% of population who can afford or % people who are employed)
2/ Income level segmentation (per year: <10,000 USD; 10,000-30,000; 30,000-50,000; 50,000-100,000; >100,000)
3/ Spending per cap per segmentation
4/ Multiply each segment population and spending per cap, sum all of them up