"We've gone back to several sources and again they have told us that as far as they are aware there was certainly a ransom paid and a figure that is being bandied around in Kabul is about $20m ... All our sources tell us that money did change hands."
He also reported that kidnappings by the Taliban were likely to continue.
"In a vow to continue with the kidnappings they [the Taliban] said that 'we will do the same thing with other allies in Afghanistan because we found this way to be successful'," he said.
The hostages were relased in separate groups on Wednesday and Thursday and reunited at the hotel in Kabul.
Two of the former hostages recounted their ordeal before they left for Dubai on Friday.
They said the Taliban kidnappers posed as passengers to seize their bus as they were travelling between Kabul and the southern city of Kandahar.
The bus had picked up two locals along the way and one of them had pointed his gun at the driver and told him to stop, 55-year-old Yu Kyeong-Sik said in comments reported by South Korea's Yonhap news agency.
"When the driver ignored him, he opened fire and the bus stopped. The Taliban told him to move the bus to the side and fired a bullet into a tyre. Then two armed men came up and beat the driver and told all of us to get off."
Yu said he and another man were driven away on a motorbike for about 10 minutes along an unpaved road to a village. The others were brought along later.
The 23-strong party was then split into five groups. Yu said his own group changed places 12 times, moving from village to village at night by motorbike or on foot.
Two female hostages had been released earlier in what the Taliban called a "gesture of goodwill" during talks with the South Korean delegation.
Two colleagues who were captured with them on July 19 had already been shot dead.Military withdrawal
The remaining hostages were freed after Seoul restated its decision to withdraw its small military presence in Afghanistan - about 200 people comprised mainly of medical workers and engineers - by the end of the year.
It also agreed with the Taliban that it would ban missionary groups from going to Afghanistan, prompting criticism from the Korea World Missions Association.
"Korean churches cannot help expressing deep concerns over the agreement reached between the government and the Taliban to halt missionary activities in Afghanistan," the organistaion said in a statement.
Shim Jin-Pyo, the father of one of the hostages who was killed by the Taliban, asked: "How could the Taliban kill him and throw his body on the street like a dog?"
"He was only trying to help children who had suffered in the war," he told Al Jazeera.
Critics have also said that the deal was a propaganda victory for the Taliban.
Rangeen Dadfar Spanta, the Afghan foreign minister, said on Germany's RBB radio that "if the impression is created now that the international community and the Afghan government allow themselves to be blackmailed, then this sends a very dangerous message".
German politicians also criticised South Korea's handling of the crisis.
Angela Merkel, the German chancellor, said Berlin would stand firm in its refusal to negotiate over a German engineer captured by the Taliban more than six weeks ago.