For "excessively expensive or luxurious":
* Lavish
* Opulent
* Profligate
* Splurging
* Extravagant
* Sumptous
* Luxurious
* Exorbitant
* Costly
* Pricey
* Prodigal
For "excessively elaborate or showy":
* Gaudy
* Over-the-top
* Outlandish
* Showy
* Flashy
* Ostentatious
* Grandiose
For "excessively wasteful":
* Wasteful
* Prodigal
* Improvident
* Thrifty
* Spendthrift
* Indiscreet
For "beyond what is reasonable or necessary":
* Excessive
* Unnecessary
* Outrageous
* Unreasonable
The best synonym for "extravagant" depends on the specific context in which you are using it.