Depends on pricing structure:

• If there's only one price, then it's just multiplying the volume by the price. Done. 
• If there are multiple prices, then you have to find a way to multiply those prices by their respective volumes. In most cases, it will be a price that corresponds to a segment, and therefore it is quite direct. But it may be different.

Let's say you have to estimate the movie theater's revenue. You may consider an average ticket price… or you may consider different prices for kids / seniors vs. adults. So you have to estimate the number of tickets for kids vs. adults, and then apply the different prices. 

If you think about cars volume, it's not difficult to estimate how many cars are sold every year. But if it is by value then you need to segment according to price (compact, mid-size, full-size) and estimate the number of cars sold, and then multiply by the respective price.

In short: if it is a “commodity” with a single price (or it is sensible to use an “average” price), just multiply by price; if there is price differentiation, (in general) you need to estimate volumes according to respective prices. 

(edited)