Jisho

Without drilling too far down into it, 代金 is the money itself used to pay for something, while 値段 is the value that someone has to pay for something. It's the difference between the actual money exchanged and the amount of money demanded. The English glosses used definitely don't make that too clear, but if you look at the Japanese definitions you can see the difference.

代金 - 品物の買い手が売り手に支払う金
値段 - 売買されるときの金額。あたい。