Cost of Living:
It doesn't really matter what the exchange rate is unless you're arguing it down significantly like this one: Although this estimate of the value of a credit suggests current average GDP capita is about $9000
https://screenrant.com/mass-effect-credits-worth-model-ships-fish-money/ Bloody inflated prices in tourist traps am I right

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However we can sort of estimate based on what we think it would take to support an interstellar empire. And yeah I kinda agree, I don't think the SA military could be supported on a GDP per capita of $55,000 so the 1 to 1 exchange rate probably doesn't work. That makes the situation worse as we're still in the position that there was no growth in the colonies so there difference must occur in the 30% of the population.
Technically you are correct it doesn't say they haven't economically advanced... but if they have economically advanced, why haven't they used that economy to purchase more up to date technology? Technology and economy are pretty intricately linked.
And if they got moved up to 1990's tech while the developed world experience 200 years worth of growth then that's honestly worse than my estimate. As at just 3% growth over that time period would expect the economy to grow 370 times in that period while the developing ones, doubled(?) tripled(?).
I don't even know where you'd put them currently to make that estimate but that would put wealth disparity at something like 100:1 rather than 20:1 assuming that they had an equal starting position! However if you like we can use this to calculate an exchange rate assuming you don't object to my initial conditions
% of world population | GDP per capita at Current Date | GDP per capita at ME Date |
70 | $ 10,000 | $ 43,219.42375 Averaging 5% growth and 30 years of advancement |
30 | $ 35,000 | $ 12,927,453.53 Averaging 3% growth and 200 years of advancement |
World average | $ 17,500 | $ 3,908,489.656 |
Since we know that in that case average gdp is about equal to 55,000 credits that gives an exchange rate of 71 dollars to the credit. After adjustment the average wealth of the 70% is now about 600 credits instead of 10,000.
I doubt your scenario however, your own source talks about slums
Seriously the only way that this lunacy gets resolved is Tri says "no, actually. all countries in the world experienced reasonable amounts of growth.". Why would we even expect developing countries to not have experienced development in the interim period? Literally the only reason I can give is massive exploitation and suppression by the developed world.