What determines the monetary value of a good or service? why are some necessities less expensive than the unnecessary items?

The general theory relates to supply and demand. In such that the costs associated with the overall supply, including managing, transporting and manufacturing of the goods, along with the demand the amount of goods or services required by a given population. When a supply is plentiful and there is less demand, the good or services general decrease in cost. When there is less supply, than there is demand, the good or service increases in cost. So If a necessity is less expensive than an unnecessary item, it could be based on the simply supply and demand issue. Or it could be related to the fundamental costs stated previously, in that in order to manufacture and/or produce the goods or services for the necessity it costs less and there is a high supply to meet the high demand, where there is a low supply to meet a low demand for the unnecessary product or service.