Money,
The nominal range with all accessories on is 13.2-14.2V. If you are reading 11.8-12.2V your battery is not getting charged. You probobly already know that though. What you probobly didn't know is that you ground and positive cables could be at fault and that is about a 50% chance roughly. Check the integrity of those first as well as the mega fuse which is located near the firewall on the back of the wiring harness central section right under the ignition module on Duratec 2.5L. If those both look and test fine then proceed with alternator removal and get it tested before you buy it. If at all possible, remove the voltage regulator and check the brushes for dirt, oil, dust, and corrosion. You may be able to check out cheap without having to buy a complete unit. Voltage regulator is $30 compared to $130 at Auto Zone.
If you do all this and still have no luck, check your wiring harness. Those aren't cheap from Ford either..
IDEA.. find one used and test it first if needed!
