"I assume that even the poorest congs in the third world buy literature at the same volume and price as the richer ones do but are they able to pay the pledged amount at $200 p/m? "
You must be joking. In countries where people are lucky to make 1$ a day per family, close to nothing is donated. Their KHs are sponsored by the wealthier countries. Again, generalizations based on wealthy, white congregations or areas are worthless. I live in an economically average, racially mixed area - in the Western world - and the monthly donation all included is some 400$.