A quick way to calculate an acurate shopping list!
1) Lets say you like the hideout named "jungle". Download the hideout and import that in the game. After that you'll get a system message from Path of Exile saying: "could not import one or more hideout doodads, hideout template imported successfully". If you don't get that message you probably already own all the decorations.
2) After importing "jungle", hit the export button and save it as a new file and name it for example "jungle_check".
3) Go back to this page and select the "jungle_check" hideout file from your pc. After that click the "Calculate" button.
4) Now you will see an acurate shopping list for the costs and decorations that you still need to buy within the game. Repeat the first 3 steps when you have bought new decorations to update the shopping list.