for the most part, Ford stock stereo systems are pretty good. that stock 6cd had 4 internal amps, putting out 20 watts rms x4, and the guts are pioneer. waay better than most aftermarket units in terms of wattage, etc- most midrange ones now are about 16-18 watts. The design of the stock cd changer, though, is pretty archaic, and prone to mechanical failure.
But yes, the dealer would probably want around $300 for a tremor head unit. which is insane. my 6cd with pioneer 2 way speakers all around kicks pretty hard for my tastes, but it won't be setting off any car alarms