I think my best nominations are all from my bookshelf, although I'm cheap so they're expensive only in a relative sense. My $80, 16-volume (and these are big, big volumes) 1913 11th edition Encyclopędia Britannica (with 1924 12th edition supplements), perhaps. My copy of The Stray Shopping Carts of Eastern North America or my accidentally autographed copy of Collectible Spoons of the Third Reich? More convincing if they weren't ten bucks each.
I think the cake goes to my two-volume, $50 two-volume Dictionary of Upriver Halkomelem. Why, my friends? Well, because it was huge, it was massive, it looked great on my bookshelf, it was from a language family I find fascinating.
I'm not very decadent, I'm afraid. My clothes are mostly thrift-store purchases, 'tho as my sense of fashion evolves, I may have to get myself some white seersucker suits since I'm moving to Oklahoma.
I did once look into buying a pickelhaube, but they cost about $150 and people will accuse you of being a Nazi.
My family have a good number of inherited antiques that would probably be viewed as decadent when purchased- we keep our napkins in a 16th century German chest, and have a sketch portrait of Teddy Roosevelt autographed by the prez himself. Alas (or fortunately, seeing what my grandmother has done with her share), while the antiques have been passed down, the money that bought them is long gone.