any cheap DAC for my Airport Express?

You should also be on the lookout for a used Assemblage DAC 2. . .this was the piece of gear that got me started in high end (and still resides in one of my systems, being fed by a 5 disc carousel and yes, an Airport Express).

Bndtracey, if your amp/receivers accept direct digital input (and I assume that to be the case based on your comment), then you are correct--a DAC is not only not necessary, but would probably introduce some more ADC and DAC steps. Also, depending on the manufacturer, the onboard DAC may be well better than the Airport Express' onboard circuitry.

If you only have analog inputs then having a DAC is still not required, BUT may be desired.

This is because an outboard DAC is likely far better quality than the onboard DAC in the AE. I use AEs in both configs. My Meridian h/t processor has optical digital input, and I go straight in and use the DAC this is onboard the processor. This is preferable since Meridian gear converts all analog inputs to digital anyway, thereby avoiding two more "conversion" points.

For my analog two channel rig, I go from the AE to an Assemblage DAC and then to my BAT pre-amp. I could go from the AE to the BAT via analog interconnect but I'm sure my Assemblage is way superior to the AE dac. Hope this helps.