If they need to evacuate US citizens abroad, they will charge the full commercial rate. That's a US government document, but let me focus on some key bits:
Who will pay to evacuate me and my family?
If commercial carriers are operating, the Embassy will recommend that
you depart on your own and at your own expense. If you don’t have money
to pay the fare, the Embassy can help you contact family and friends and
assist in transferring money from them. In extreme circumstances, a
destitute American wishing to return to the U.S. may qualify for a
repatriation loan. The conditions for making such loans are stringent,
and your passport will be limited until you repay the loan.....<read more> http://www.overseas-exile.com/2013/03/us-expat-evacuations-not-what-people.html
