Use the noun extravagance when you're talking about something that's over the top, especially when it comes to spending money. Ordering a $500 hamburger for dinner would be an extravagance.
Money is usually the subject when people talk about extravagance, although the word can also mean having too much of something, whether it's anger or flowery wallpaper. In Latin, the root word extravagari means "wander outside or beyond," and originally extravagance was used to describe something that was unusual — "wandering outside" the norm. It wasn't until the 1700s that the word became associated specifically with spending too much money.