“Not a dime’s worth of difference” means that there is no difference. When comparing two products, one might cost a dime more; the more expensive product is expected to be better or to have more features. “Dime’s worth of difference” is cited in print since at least 1939. “Dollar’s worth of difference” is also sometimes used.
 
George Wallace (1919-1998) ran for U.S. president in 1968 on the third party American Independent Party ticket. Asked in 1968 about the Democrats and Republicans, Wallace answered: “There’s not one dime’s worth of difference in the two parties. If you put all of the Republican leaders in a good cotton-picking sack with the Democrats, shake ‘em up and let the first one drop out, you’d stick ‘im right back in ‘cause there’s no difference.” Wallace had used the “not a dime’s worth of difference” line as early as May 1967.
 
Advocates for a third political party in the United States often use the “not a dime’s worth of difference” line, disassociating the phrase with the policies of George Wallace or the depreciation of the dime. The “not a dime’s worth of difference” phrase has also been used to describe similar candidates of the Democrat party or Republican party in primary elections.
 
 
Wikipedia: George Wallace
George Corley Wallace, Jr. (August 25, 1919–September 13, 1998), was a governor of Alabama for four terms; 1963–1967, 1971–1979 and 1983–1987. “The most influential loser” in 20th-century U.S. politics, according to biographers Dan T. Carter and Stephan Lesher, he ran for US president four times, running officially as a Democrat three times and in the American Independent Party once. A 1972 assassination attempt left him wheel chair-bound. He is best known for his Southern populist pro-segregation attitudes during the American desegregation period, convictions he renounced later in life.
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1968 third party presidential run
Wallace ran for US president in the 1968 election as the American Independent Party candidate. He hoped to force the House of Representatives to decide the election by receiving enough electoral votes, presumably giving him the role of a power broker. Wallace hoped that southern states could use their clout to end federal efforts at desegregation. His platform contained generous increases for beneficiaries of Social Security and Medicare.
         
8 November 1925, Cleveland (OH) Plain Dealer, pg. 4C, col. 1:
As between the two types of houses and the lots upon which they will be erected there will be not a dollar’s worth of difference as to value.
   
Google Books
Pump-priming theory of government spending
By Egbert Ray Nichols and William E. Roskam
New York, NY: H. W. WIlson Company
1939
Pg. 111:
Charges of politics in relief take some of the bloom off the flower of recovery efforts, but they don’t make a dime’s worth of difference in the huge turnover of dollars in wages and sales.
 
Google Books
Here Comes Labor
By Chester Maynard Wright
New York, NY: THe Macmillan Company
1939
Pg. 121:
There isn’t a dime’s worth of difference in their handling of negotiations and agreements.
     
Google Books
Waste Heritage
By Irene Baird
New York, NY: Random House
1939
Pg. 146:
“Don’t thank me. If that woman had hared off to the police, then the whole thing would have to come out an’ nothing I could have done would have made a dime’s worth of difference.”
 
Google Books
Purlie victorious: a comedy in three acts
By Ossie Davis
New York, NY: Samuel French
1961
Pg. 25:
PURLIE. Remind this white and wicked world there ain’t been more’n a dime’s worth of difference twixt one man and another’n, irregardless of race, gender, creed, or color—since God Himself Almighty set the first batch out to dry before the chimneys of Zion got hot!
   
12 May 1967, Trenton (NJ) Evening Times, pg. 2, col. 3:
Wallace predicted that his possible candidacy would hurt both parties and not only one. “If both parties continue in their liberalistic trend,” he said. “It doesn’t matter which you hurt, they’re both the same—there’s not a dime’s worth of difference between the candidates now being discussed.”
 
Google News Archive
4 July 1968, Pittsburgh (PA) Post-Gazette, “Wallace Followers, Foes Clash,” pg. 25, col. 1:
MINNEAPOLIS, July 3 (AP)—Police used night sticks and Mace to restore order tonight when demonstrators clashed with a crowd gathered to hear a speech by former Alabama Gov. George C. Wallace.
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Queries about his rating of the top contenders for the Republican and Democratic nominations, Wallace replied: “Politically, they’re all about the same. There’s not a dime’s worth of difference between them.”
 
Google News Archive
30 July 1968, St. Petersburg (FL) Times, “800 At Wallace Dinner” by Ron Hitchison, pg. 1B, cols. 1-4:
TAMPA—Former Alabama governor George Wallace, shortly after filing a suit challenging an election law in Ohio, predicted he will be on the ballot in all 50 states in November.
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“What’s the difference in the national leadership of the Republican and Democratic parties,” Wallace asked. “There’s not one dime’s worth of difference in the two parties. If you put all of the Republican leaders in a good cotton-picking sack with the Democrats, shake ‘em up and let the first one drop out, you’d stick ‘im right back in ‘cause there’s no difference.”
 
Time magazine
Essay: WHAT IF YOU DON’T VOTE?
Friday, Nov. 01, 1968
(...)
Across the country, the abstainers are mainly disaffected McCarthy and Kennedy Democrats, plus some Rockefeller Republicans. If they agree with George Wallace on nothing else, many still feel that “there’s not a dime’s worth of difference” between Nixon and Humphrey.
 
OCLC WorldCat record
When can voters tell if there’s a dime’s worth of difference? : modeling voter uncertainty about candidate issue placements
Author: R Michael Alvarez
Publisher: [Durham, NC] : Duke University Program in Political Economy, [1991]
Series: Papers in American politics; Working paper (Duke University. Program in Political Economy), no. 147. 
Edition/Format: Book : English
 
OCLC WorldCat record
The ten things you can’t say in America
Author: Larry Elder
Publisher: New York : St. Martin’s Press, 2000.
Edition/Format: Book : English : 1st ed
Contents: Blacks are more racist than Whites—White condescension is as bad as Black racism—The media bias—it’s real, it’s widespread, it’s destructive—The glass ceiling—full of holes—America’s greatest problem: not crime, racism, or bad schools—its illegitimacy—There is no health-care “crisis”—America’s welfare state: the tyranny of the statist quo—Republicans versus Democrats—maybe a dime’s worth of difference
   
OCLC WorldCat record
Dime’s worth of difference : beyond the lesser of two evils
Author: Alexander Cockburn; Jeffrey St Clair
Publisher: Petrolia, Calif. : CounterPunch ; Oakland, Calif. : AK Press, 2004.
Edition/Format: Book : English