Cheapest investment in Africa
Cheapest investment I can think of is planting fruit trees e.g. Mayuni/Avocado. It takes about 5 years before it starts yielding fruits. Cheap assuming land is free and water is close or their is irrigation system incase of rain shortage within the first five years. To acquire seeds the formal way is to go to local seedling store but the not common way is the buy actual fruits during the season from wherever you get it. Plant about hundred trees just around your home or a piece of land then fence it. While it grows, begin to think about how you will eat the fruit, with what you will eat the fruit and educate people about your thinking around. The other advantage about investment is number of people would have grown through migration to your area by birth because on average in 5-6 years, youthful generation would have had two children each and if there are 20 couple of such, you now have 60 people. Trees takes so many years growing hence after five years, it only produce more fruits and the yields only goes up. Five years after the first yield for a hundred trees, you should produce enough fruits for the village and others around including your town. By that time, you already have enough customers you only waits for the next season.