In Pokémon Platinum, when the player arrives at the Resort Area, he/she will be shown into a house of his/her own. It can be then used similarly to a secret base. It will, however, also be visited by others like the Sinnoh Gym Leaders, Professor Rowan, his assistant, the player's rival, and the player's mom.
The stuff is expensive because it's Resort Area. Resorts are fairly expensive.