Creating a virtual world often means putting money into development for months or years. We need to earn from the virtual world after deployment in production to cover the cost. There are several common ways to earn money from the virtual world.
Google AdSense is the advertising system for web publishers. AdSense for Content will display advertising on the website of the virtual world. We earn money when the players visit the virtual world website and click on the advertisement.
Instead of advertising on the website, we can put ads inside the game. It can be a billboard on some advertisement painting on the wall of the buildings. It can also be a virtual item. Imagine that drinking a Coke can recharge the health point quicker than drinking water. Then we find a Coke company to pay some advertising fee to put their logo on the item.
The following screenshot shows the Sony BMG in-game advertisement from Second Life: