I've seen several advertising campaigns that promote the decadence of this or that luxury item over the years. Decadent jewelry, decadent chocolate; images meant to appeal to the prurient nature in us all, I guess, because I can't think of a positive reason for buying something marked by decay.
The root of decadent is decay; marked by decay, in decline, falling or sinking. Basically, when someone wants you to buy something decadent, they are asking you to waste your money on something frivolous or meaningless, something that is in transition to a lower order of things.
Opulence, on the other hand, is the word that most people confuse with decadence. To be opulent is to have the trappings of power or authority. To possess those things achieved through work and success. Opulence can be ostentatious, but it's never decadent.
So, the next time your tempted to buy rotting chocolate ice cream, just say "no thanks". Decay is not a good thing to invest in. You can spend your way into decadence (the federal government is proving this truism as I type) but decadence never turns into opulence. It just turns into more fertilizer.