First of all, millions of dollars have been funnelled to Syria since 2011 to support the fighters/militants/jihadists/mujahedeen that fought against the al-Assad regime. Some of the money went to other groups like Jabhat al-Nusra and groups that really are affiliated or joined the Islamic State later. The Islamic State took control of the banks and robbed the banks in Iraq. They got funds from this source. They also export oil that goes to Turkey.
According to Canadian jihadists who joined ISIS, or joined the Islamic State in Iraq, they said—I'm speaking specifically of a Calgarian, who is in Mosul right now. He said that the conditions in Iraq for those who join ISIS are excellent. They are being paid a monthly salary, and the Islamic State takes care of their families. They pay all their expenses—