The most expensive non-collectible, non-cheated, non player created item in the game unlocks at level ten of the scientist career and is the Out of This World Desktop at 15100. The next is of course The Vertuoso Violin at 15000. After that is The Schmapple Fridge 14000. Then there is multiple items at 12000. I am adding non-collectible, non-cheated, non player created items disclaimer as I never checked them can't speak for certain on their top values.
Hope this helped. If you don't mind me asking,
@Scarce, Why do you ask? Just wondering. Are you building the most exuberant house ever or something? If so please share you creation.
And
@Scarce welcome to the forum. If you have anymore questions please ask.