I would encourage that you expand this to also include people from the developed world. A small amount of money, or a gift card as a thank you, is not unprecedented. Blood banks give you cookies after you donate blood...you might get more interest if that same blood bank handed out the monetary value of those cookies ($2).
Veronica, your point is a good one. My creating the market, you are legitimizing the trade. That said, in some cases, a regulated legitimate market is often better for all involved than an unregulated black market for those same goods.
My concern regarding using 3rd world countries is that their bone marrow will be primarily benefiting the rich urban elite. If I live off the grid in a poor village in SE Asia, and I have cancer, even if my mother donates bone marrow, I will never have the means for a transplant.
That said, based on my experience in rural Cambodia, I believe that the cost to register could be much, much lower. For a simple cheek swab, people would be lining up for the equivalent of 12 cents.