I estimate it is about 1.2 cents/mile. But if it becomes very difficult to get standard or (25k domestic or 50k Europe) award tickets, then it drops to about 0.6 cents/mile. At that level it no longer makes sense to get the credit cards or fly just for miles when you have a choice on personal travel. On international travel I dont look for miles, just convenience and dates.

Airlines might tell you otherwise, they estimate at 2 cents/mile.