Value of an item is not determined by the amount of labor it takes.
Value is subjective. No matter how many hours you took to make a product, I may value it differently than another person. Its the basic comparison to a bottle of water or a 1 oz crystal diamond.
Lets say it takes 5 hours of labor to make 1 bottle of water, and 200 hours of labor to make 1 oz diamond. If you believe in labor theory of value, both items should be valued equally both in africa and america.
However in reality, here in america, if we were faced with a choice, wed choose the 1 oz diamond, since a bottle of water is so abundant that we would in a heartbeat choose a 1 oz crystal diamond, and as such i value the diamond more than the water even though both items took the same amount of labor to make.
However, in africa, a child that is thirsty will choose the bottle of water. Ddrinkable water is not very abundant in africa and as such, the african child will value the water above the diamond, even though it takes more labor to extract a diamond, than to purify water.
Hence, value comes not from the amount of labor put in it, but from its marginal utility:
http://en.wikipedia.org/wiki/Marginal_utility