A ransom or price is not a payment to the Devil to redeem his slaves but a payment to God’s justice to redeem us from sin, world, law, Devil and death. This payment is owed to Gods law due to the sin of man. This debt needs to be paid.
the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many. (The New International Version. 2011, Mt 20:28)
The price of redemption is the blood of Christ, which atones for sin.
The word ransom (λυτρον) is derived from the Hebrew verb כִּפֶּר “to atone”…The idea is not that of simple freeing from a fault, but of its recognition and expiation in the substitutionary offering. Thus when λύτρον (-α) is used for כֹּפֶר, it always denotes a vicarious gift whose value covers a fault, so that the debt is not just cancelled. Indeed, in the equation כֹּפֶר == λύτρον the offering is always for a human life (ψυχή). (Vol. 4: Theological dictionary of the New Testament. 1964- (G. Kittel, G. W. Bromiley & G. Friedrich, Ed.) (electronic ed.) (329). Grand Rapids, MI: Eerdmans.)
This ‘payment’ is therefore substitutionary ‘life for life’. Christ died to pay the penalty of our sins, and as the law also demands positive and perfect obedience, his perfect righteousness is also included in the payment. His perfect obedience is counted as ours. His obedience is charged as an alien gift to the sinners account so that they are ‘declared righteous and obedient’, even while still sinners.
While we were still sinners, Christ died for us. Since we have now been justified by his blood, how much more shall we be saved from God’s wrath through him! (The New International Version. 2011 ,Ro 5:8–10)