Sentence:  Economics is a messy discipline: too fluid to be a science, too rigorous to be an art. Perhaps it is fitting that economists' most-used metric, gross domestic product (GDP), is a tangle too. GDP measures the total <WSD>value</WSD> of output in an economic territory. Its apparent simplicity explains why it is scrutinised down to tenths of a percentage point every month. 
Meaning of the word marked with <WSD> tags:  The amount (of money or goods or services) that is considered to be a fair equivalent for something else.