lucre vs fast buck

lucre and fast buck both are nouns.

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

As nouns, fast buck is a hyponym of lucre; that is, fast buck is a word with a more specific, narrower meaning than lucre:

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

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