Under the terms of the deal, South Korea agreed to end missionary activities by Christian groups in Afghanistan.
The Taliban initially demanded the release of its members held prisoner by the Afghan government.
The group killed two of the Koreans and then released another two earlier in what it said was a goodwill gesture.
There has also been speculation that the South Koreans bought the release of the hostages, though both the Taliban and the South Korean government denied there was any secret deal.
Alan Fisher, reporting for Al Jazeera from Kabul, said: "We've certainly heard rumours around Kabul - the figure of around 20 million pounds has been bandied around.
"I spoke to one senior Afghan authority who, while not confirming the figure, did say that money was paid - that the South Koreans had paid cash to the Taliban."
But Qari Mohammad Bashir, a Taliban commander, denied that a ransom had been paid.
"I strongly deny this. It's not true that money was involved," he said.
Al Jazeera's Tony Birtley, reporting from South Korea, said: "Most people here [in Seoul] think that South Korea has probably paid a ransom, but that will be debated later when the hostages have returned home safely."
Prior to the kidnapping, South Korea warned its citizens not to travel to Afghanistan and blocked many of its growing legion of evangelical Christians from going there due to safety concerns.
In a separate development in southern Afghanistan on Thursday, a soldier with the Nato-led military force in Afghanistan and an Afghan interpreter were killed while on patrol, Nato said.
Two other soldiers were wounded in the incident, Nato's International Security Assistance Force (ISAF) said in a statement, but did not give the nationality of the foreign soldier or location of the incident.
Source: Al Jazeera and agencies