A function I could think of is the cost of buying bread. If I only buy a single type of bread, then the function is the number of bread that I buy and the output is the total cost.
Let x be the number of bread I buy
f(x) be the cost of buying a certain number of bread
The price of a single bread is 20 cents
f(x) = 0.2x
Domain: x > 0
Range: y > 0