Sternyy - I know you can buy a unit for $150 or so, but the installation is a bitch. I put one in my '96 Contour, and by the time I got done buying extras - switches, etc., I was out another $50. Plus I had to have the wiring diagram for the car (which I haven't bought for the Fusion - they aren't cheap), and tracing the wires takes a lot of time.
The regular installers know what they are doing, and do many cars, so they are fast. But for me to do it, I would gladly pay the $325 and be done with it - including having to fix it if it breaks.
Just my ideas - if you don't mind spending a lot of hours doing the work, you CAN save substantial $$.