According to the Washington Post article Selling votes is common type of election fraud:
corrupt candidates in Clay County, Ky., once paid $100. But that was probably too much: It attracted one woman who already had sold her vote. The man who bought it first was outraged, and he beat up the man who bought it second.
Is this a true story?
Maybe.
Official FBI Press Release of the convictions in 2010, there were eight defendants in total. From the Lexington Herald Leader:
In one of those cases, vote-buyer Bobby Red Sams testified that [William Bart Morris], who was working for a different candidate than Sams, beat him up in a dispute over a woman who had sold her vote.
Morris was one of the defendants in the Federal RICO case. My interpretation of the claim hinges on Mr. Sams' testimony (which I cannot find a transcript of), which is why my conclusion is "maybe". I could not find any charges against a Bobby Red Sams for perjury, or even mention of a plea deal for someone described as a "vote buyer".
Here is a link to an appellate court's ruling to vacate all of the defendants' convictions in 2013, and another article from the Lexington Herald Leader describing it. Nothing in either source discounts Mr. Sams' testimony directly. Morris, his wife, and the rest of the defendants later plead guilty to a "racketeering conspiracy" charge, but not for assault. As part of the deal Mr. Morris agreed to serve 5½ years in federal prison.
The original case was USA v. Maricle et al.
I haven't yet seen anything in any of the sources that mentions the cost of the vote, which to me seems to be a pretty small amount to be paid for breaking a federal law.
