fast buck vs lucre

fast buck and lucre both are nouns.

Word	Noun	Adjective	Verb	Adverb
fast buck	Yes	No	No	No
lucre	Yes	No	No	No

As nouns, lucre is a hypernym of fast buck; that is, lucre is a word with a broader meaning than fast buck:

fast buck: quick or easy earnings
lucre: the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)

Other hypernyms of fast buck include earnings, net, net income, net profit, profit, profits.

fast buck (noun)	lucre (noun)
quick or easy earnings	the excess of revenues over outlays in a given period of time (including depreciation and other non-cash expenses)
	informal terms for money