From the Super Mario Wiki
Bills are a kind of money that Luigi collects in Luigi's Mansion and it has the second-least value. Each one is worth 20,000G and they are usually found in groups by examining objects and moving chandeliers in the rooms with the Poltergust 3000. They resemble Canadian bills as they are multicolored. Luigi can easily collect them, as they can be sucked up by the Poltergust 3000.
Bills also make an appearance in Luigi's Mansion: Dark Moon, where they take the same role as they do in Luigi's Mansion. However, instead of being worth 20,000G, they are worth 5G.