Money is the currency Urotsuki can collect in her dreams, with the symbol 夢 (Yume, Dream).
Obtaining Money
Money (equal to the proportion of maps you have visited to all) 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 (1/8 chance). 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. Killing the blue boy in Urotsuki's Dream Apartments always gives 50夢.
A blue haired maid npc who appears on many maps will also give you 500夢 when you interact with her. This can be done once per dream.
There is a smiling green blob in Graveyard World near the Sculpture room that will give you 5000夢 if you can chainsaw it. However, doing so is very difficult, as the blob moves across the map at very high speeds.
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.
Money can also be gambled at the kura slot machine minigame in the Developer Room for profit or loss, at 100夢 per spin.
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 may purchase drinks from them for 100夢.
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. It gets cheaper 10夢 per purchase (down until 100夢).
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.