extravagant
Going way beyond what is reasonable, often wasting money.
Extravagant means spending much more money than necessary, or going way beyond what's reasonable or appropriate. When someone buys an extravagant gift, they might spend hundreds of dollars on something when a thoughtful ten-dollar present would have been just as meaningful. An extravagant party might feature ice sculptures, a live band, and a five-tier cake when simple decorations and homemade treats would have made everyone just as happy.
The word can describe behavior, spending, or even claims and statements. An extravagant promise is one that goes too far, like guaranteeing you'll become a millionaire if you follow some simple steps. An extravagant compliment might be so over-the-top that it stops sounding sincere.
Extravagance (the noun form) suggests wastefulness or excess. There's a difference between being generous and being extravagant: generosity comes from wanting to help or delight someone, while extravagance can involve showing off or not thinking carefully about resources. When your school fundraiser sets an extravagant goal of raising a million dollars instead of a realistic five thousand, that's extravagance in action.