Expiation for not praying
Question

                What is 'Kaffara'? What price should have to be given for 'one raka/ oakt salah' as kaffara?.
Praise be to Allah.Whoever omits one or more obligatory prayers 
    with no excuse has to repent to Allaah sincerely, and he does not have to make them up or offer any expiation (kafaarah), because omitting the 
    obligatory prayers deliberately is major kufr, as the Prophet  (peace and blessings of Allaah be upon him) said: “The covenant that stands 
    between us and them is prayer; whoever gives it up is a kaafir.” Narrated by Ahmad, 22428; al-Tirmidhi, 2621; al-Nasaa’i, 462. And he said: 
    “Between a man and shirk and kufr there stands his giving up prayer.” Narrated by Muslim, 242. There is no expiation for that except sincere 
    repentance. See Fataawa al-Lajnah al-Daa’imah, 6/50 
Prayer is an act of worship that is to performed at specific times, and whoever does not do an 
    act of worship that is tied to a specific time – such as prayer and fasting – until that time is over, with no excuse, then repents, does not have 
    to make up what he omitted, because this act of worship has been limited by the Lawgiver to the beginning and ending of that time. 
Fataawa al-Shaykh Ibn ‘Uthaymeen, 
    1/322. 
But if a person misses a prayer for a valid 
    excuse such as sleeping or forgetting, his expiation is to offer the prayer when he remembers it, and there is no other expiation required, as the 
    Prophet  (peace and blessings of Allaah be upon him) said: “Whoever forgets a prayer or sleeps and misses it, let him pray it when he 
    remembers, and there is no other expiation for it but that.” 
Al-Bukhaari, 572; Muslim, 1564 
And Allaah knows best.