The challenge of assessing the size of an emerging market opportunity

Kagio fresh produce market, Kenya, April 2013

Untapped opportunities in the developing world bring with them their own challenges for businesses seeking to invest in them.  An interesting one is that of assessing the market size and value, particularly for the lower income demographic that operates primarily in the informal economy (often called the BoP or bottom of the pyramid).  This snippet frames it well:

To begin, it is critical to understand why traditional market sizing methodologies are ill-equipped to size emerging markets. To illustrate, if a firm were to use traditional methods to size a mature market such as the coffee market in the United States, it would consider demographic trends (e.g., aging baby boomers), psychographic trends (e.g., increased health consciousness), past sales trends and consumption rates, price movements, competitor brand shares and new product development, and channels/retailers among others. However, conducting such an analysis for emerging markets presents a challenge as several of these factors (e.g., past sales, demographics of the customer when there are no current customers) don't exist because the markets are presently untapped.

The situation is exacerbated  by lack of easily available demographic data, few formal retail channels, little consumer knowledge, and if the majority of the target audience happens to be outside the urban population centers, even lack of basic infrastructure like roads. One must begin from scratch. Can any rules of thumb be developed as we increase our understanding of the next few billion customers?
This conversation will continue.