The regular door and deck speakers are just 25 watterís, with the HU delivering about 22 watts max to each of those. However the door woofers are special 1 ohm dual voice coil units. The shaker amps are high current devices that are designed to drive those ridiculously heavy loads with 10 amps or more. So the door woofer power gets up to around 100 watts each side (I havenít had the parts set up on the bench to get exact numbers). With the unique design of the woofer/amp combination if you substitute one you have to substitute the other too, i.e. the shaker amps wonít deliver any more power into a standard 4 ohm woofer than the HU could. $7,000 sounds nuts though, no matter what they do. You donít mention what year your pony is, but it may only blue book for a few thousand more than that total.
Sorry I forgot to mention that, it's a 2007 Mustang GT! Also, by the sounds of it I would be better off going the aftermarket everything route? Also, have you ever had an issue removing the amplifier and replacing with an aftermarket one? If so, where did you mount it?