“If you lend someone $20 and never see that person again, it was probably worth it” is a popular joke, meaning that the $20 (that is not paid back) is a small sum to be rid of an untrustworthy person. “If you lend someone $10 and don’t see him again, it may be worth it” was cited in print in 1948. “If you lend someone a ‘fiver’ and he avoids you — you’ve gotten off cheap” was cited in print in 1951.
 
The original author of the joke is unknown. The “$20” amount has been cited in print since the 1990s.
 
 
22 May 1948, Marion (OH) Star, pg. 2, col. 7 ad:
“If you lend someone $10 and don’t see him again, it may be worth it.” 
(Marion Loan Co.—ed.)
 
10 February 1949, Emporia (KS) Weekly Gazette, pg. 2, col. 2:
If you lend someone $10 and don’t see him again it may be worth it, says the Lyons News.
 
Google Books
Typo Graphic
1950
Pg. 252:
If you lend someone $10 and never see him again, it may be worth it.
 
Google Books
A Treasury of Jewish Humor
By Nathan Ausubel
Garden City, NY: Doubleday
1951
Pg. 599:
If you lend someone a “fiver” and he avoids you — you’ve gotten off cheap.
 
16 January 1967, Aberdeen (SD) American-News, “Poor Man’s Philosopher” by Hal Boyle, pg. 4, col. 3:
Worth remembering: “If you lend someone $5 and never see him again, you got a great bargain.”
 
26 March 1999, The Pantagraph (Bloomington, IL), “Men? Equal? It all depends just who you ask”:
“If you lend someone $20 and never see that person again, it was probably worth it.”
 
8 June 2000, August (GA) Chronicle, “Think before picking up that calculus book” by Glynn Moore, Richmond County Neighbors, pg. 2, col. 5:
If you lend someone $20, and never see that person again, it was probably worth it.
 
Google News Archive
18 September 2000, The Blade (Toledo, OH), Ann Landers column, sec. D, pg. 8, col. 5:
Gem of the Day (Credit David Broome in Mesa, Ariz.): If you lend someone $20 and never see that person again, it was probably worth it.
 
Google Books
Read This; It’s funny!!
By Carl E. Knox
Portland, OR: OCGI
2003
Pg. 178:
If you lend somebody $20 and never see that person again, it was probably worth it.
 
Google Books
Wise Quotes of Wisdom:
A Lifetime Collection of Quotes, Sayings, Philosophies, Viewpoints and Thoughts
By R.A. Wise
Bloomington, IN: AuthorHouse
2011
Pg. 108:
If you lend someone $20 & never see that person again, it was probably worth it.
 
Google Books
Life’s Daily Dose of Wit & Wisdom:
Wit & Wisdom to Enlighten Your Day
By J. N. Hospitable
Bloomington, IN: AuthorHouse
2011
Pg. 176:
If you lend someone $20 and never see that person again, it was probably worth it. (Unknown)