The cost of one type of material does not necessarily equate to the cost of the other. There are a number of things that factor into the final price tag for materials such as installation, tool use, storage space and other factors.
What separates wood from plastic decking is that wood offers many more design choices than plastic decking does. One would need to consider how these factors work with their overall budget and situation in order to make a choice on which material they would prefer for their project or home improvement!