One Box CD Units vs Transport & DAC

As a result of various Audiogon forums, I have managed to over the last 2 years put together a respectable home theatre. Setup includes Cal Audio 2500 Gear for the DVD, Amp, & SSP. Aerial 10 fronts, Vienna Accoustics Center, Rel Sub, & Cardas Gold cableing.

Currently the Cal DVD is pulling double duty as a CD player.
I listen exclusively to Redbook material.

The question I have is regarding DAC's. Why are DAC's that are 4 to 5 years old sell at 25 to 30 percent of their original sales price while CD players in the same age range seem to be holding 50 to 60 percent of their sales price. This type of pricing disparity makes it tempting to get one of the nicer DAC's, (Classe, Wadia, Audio Research) and utilize the DVD player as the transport. Overall however if one were building a system from the ground up, would be interested in hearing opinions of a transport/DAC route versus one Box CD players. Put another way with a budget of $1,500 for used equipment, whats going to deliver the biggest bang for the buck. Thanks.