Decided to look up some definitions on my beloved Wiktionary and found something interesting.
The third defintion of the word "luxury" is given as: "Something that is pleasant but not necessary in life."
This seems to fit perfectly with the Epicurean view of desires that are natural but unnecessary.
On the other hand, the defintions given for the word "extravagance" are markedly anti-Epicurean, aligning more closely with empty/corrosive desires, such as "excessive" and "prodigality".
I realise that dictionary defintions rarely represent concrete, objective meanings, but perhaps they can be useful in pointing us towards how words are most commonly used and interpreted.