Answer: TIME.
Explanation: Element of value can be said to be one of the simplest or essential parts or principles of anything that renders something desirable or valuable.
In this example, the element of decision that affected her decision is TIME. Here, time is a valuable asset such that the place where she can get cheaper soda is a bit far and as time is desirable and valuable to her, she chose to get it nextdoor where it is will take less time but more expensive.