Study Watchtower, p. 10 par. 6 - "A ransom is a price that is paid to buy something back.
In its simpliest biblical context, it means simply the price of freedom.
Websters - 1828 Edition
Ransom
RAN'SOM, noun
1. The money or price paid for the redemption of a prisoner or slave, or for goods captured by an enemy; that which procures the release of a prisoner or captive, or of captured property, and restores the one to liberty and the other to the original owner.