Surprisingly enough, you can use dawn dish soap and a downy or bounce dryer sheet and the sap will come right off. If you went to the auto detail shop to get an estimate they might tell you something like $400-$900 to remove all the sap. You can do it yourself real quick with this method.

There are many products out there like Goof or evening rubbing alcohol, but hardened sycamore tree sap on automobile paint, dawn dish soap, and a dryer sheet work like magic.