I was hoping that this information is in the Quran but I could not find it. Apparently the Quran only says, Pray Salat and give Zakat. Where does this value come from? Are there any variation in it among different sects?
This a good question. Here are some hadith to prove the percentage.
Classed as saheeh by al-Albaani in Saheeh Ibn Maajah.
Ibn Abi Shaybah narrated in al-Musannad (9966) with a jayyid isnaad from ‘Ali (may Allah be pleased with him) that he said: Nothing is due on anything less than twenty dinars; for twenty dinars, half a dinar is due; for forty dinars, one dinar is due. For anything more than that, it is to be worked out. Irwa’ al-Ghaleel, 3/291
Saheeh Abi Dawood.
Ibn Maajah (1791) narrated from Ibn ‘Umar and ‘Aa’ishah (may Allah be pleased with them both) that the Prophet (blessings and peace of Allah be upon him) used to take from every twenty dinars or more, half a dinar, and from forty dinars, one dinar.
From the above hadiths, the percentage is 2.5%..
For detailed answer, please consult: http://islamqa.info/en/145600