Money is the currency Urotsuki can collect in her dreams, with the symbol 夢 (Yume, Dream).
Obtaining Money
Money (81夢) can be obtained once per dream by interacting with the piggy bank at the foot of Urotsuki's bed.
Killing NPCs can sometimes yield 100夢. The NPCs in Ecstasy World seem to give out money more often than NPCs in other areas, making this a good place to get money quickly.
You can find some money by checking the Box Shop Keeper's drawers in Japan Town and going into the gap in the hedges near the Underground Bar. (In recent versions, this may not occur anymore.)
Once per game, Urotsuki can receive money from a customer in the hotel.
Spending Money
There are currently only a handful of things Urotsuki can spend money on. The many vending machines found throughout the dream world are the easiest to access.
Urotsuki can also buy alcoholic drinks (200夢 per drink) at the Underground Bar in Japan Town. Drinking too much may lead to interesting results.
Masks can be purchased from the shopkeeper in the Mask Shop for 500夢 each.
Urotsuki can also lose money. The vending machine in Mushroom World, for example, will steal your money and not give you a drink. A sane clown (nicknamed Pennywise) in the gray building of Guts World will also steal some money from you if you try to kill him.