The reward for catching all 50 Boos is a large diamond.
Here are th e positions of the money ghosts:
Wardrobe Room - hiding inside the right-hand cupboard.
Study - check the whole of Neville's desk.
Storage Room - tap at the chair on the right.
Dining Room - inside the left-hand cabinet.
Kitchen - inside the left-hand oven.
Rec Room - inside the training machine to the bottom-right.
Nana's Room - inside the righ-hand table.
Billiards Room - search the bottom-right chair.
Twin's Bedroom - on top of the bunk bed.
Breaker Room - underneath the tablecloth.
Cellar - inside one of the barrels.
Second floor Treasure Room - inside one of the chests.
Basically, any mirror in the mansion (except for the large one that reflects the invisible ghosts) can act as a warp. To warp using a mirror, switch to your Game Boy Horror's first person mode and examine the mirror with A. After a really trippy warp sequence, you will find yourself in the entrance hall of Luigi's Mansion.
Note that if you fail to return and water the plant after a boss fight, the plant will whither and die.
The first one is located on the first floor, in the south-west corner of the Mansion. After you have sucked in the candle-carrying butler, exmaine the room with your Game Boy Horror to find a mousehole. Press A to activate it and go inside to enter the trasure room.
The second treasure room is accessible from the roof top. Use the elevator in the toy chamber to get on the roof, the go all the way to the right and climb up on the chimney. Fall down and you will land in another treasure room.