A quantity value is comprised of a number and a unit of measure, e.g. 102 345.34 m³ or 98 000.9 bbl. It is associated to a bulk product which it characterizes, typically determined by a measurement or a quantity conversion from a measurement quantity value. You multiply a quantity value with the price per unit of measure for that product to obtain the value of the product.
Diesel is e.g. priced at 500 € / m³ at 15 °C.
You multiply: 102 345.34 m³ × 500 € / m³ = 51 172 670 €, which is the value of your product.
Since volumes change with temperature, it is important to convert the m³ quantity value from an observed volume to a m³ quantity value for a volume at a defined reference temperature.