Two thousand Nigerian naira equals $12.45 US dollars. This articles is a bit unclear, as it says that the jihadis bid on the girls, but also states that this amount was a “dowry” that was paid to the captors of the girls. Possibly the twelve bucks and change was an average price.
In any case, kidnapping infidels and either killing them, enslaving them, ransoming them or exchanging them for Muslim prisoners, or releasing them outright is fully sanctioned in Islamic law: “As for the captives, the amir [ruler] has the choice of taking the most beneficial action of four possibilities: the first to put them to death by cutting their necks; the second, to enslave them and apply the laws of slavery regarding their sale and manumission; the third, to ransom them in exchange for goods or prisoners; and fourth, to show favor to them and pardon them. Allah, may he be exalted, says, ‘When you encounter those [infidels] who deny [the Truth=Islam] then strike [their] necks’ (Qur’an sura 47, verse 4)” — Abu’l-Hasan al-Mawardi, al-Ahkam as-Sultaniyyah (The Laws of Islamic Governance), trans. by Dr. Asadullah Yate, (London), Ta-Ha Publishers Ltd., 1996, p. 192.