tidy sum vs plenty

tidy sum and plenty both are nouns.

Word	Noun	Adjective	Verb	Adverb
tidy sum	Yes	No	No	No
plenty	Yes	No	No	Yes

As nouns, tidy sum and plenty are synonyms defined as:

tidy sum and plenty: (often followed by `of') a large number or amount or extent

tidy sum (noun)	plenty (noun)
(often followed by `of') a large number or amount or extent	a full supply
	(often followed by `of') a large number or amount or extent

tidy sum (adverb)	plenty (adverb)
	as much as necessary