Final answer:
To model costs, mathematical expressions are used to reflect variables like unit cost and quantity; as for opportunity costs, they represent the value of the best alternative forsaken. True opportunity costs may include non-monetary factors like time.
Step-by-step explanation:
To model the cost of an item, various mathematical expressions can be used based on the specifics of the situation. A mathematical model for the cost could involve considering all the variables that influence the final price, such as the unit cost, quantity, taxes, and discounts. For example, if we're looking at water costs for different systems, one might use the expression C = p × q + t - d where C represents the total cost, p is the unit price of water, q is the quantity of water used, t is the taxes applied, and d is any discounts provided.
When discussing opportunity costs, the expression for cost may need to reflect the value of what is given up. If spending $300 on a bicycle, the opportunity cost is the other consumption forsaken, such as books or music. Hence, in an economic model, the expression might look like Budget = money_spent_on_item - money_spent_on_alternatives, assuming these are the only expenditures.
It's important to remember that sometimes the true opportunity cost cannot be sufficiently expressed in monetary terms, especially when time or other non-quantifiable factors are involved. Therefore, a comprehensive cost model might need to combine both quantitative and qualitative assessments of costs.