any cheap DAC for my Airport Express?

If you really are looking for something cheap that will give you a good bang for your buck you can check out ebay or here for an Entech Number Cruncher DAC. they're older, but you can sometimes still find new old stock. The 203.2 often can be had for $50 or less and the 205.2 typically goes for $100.

I use my Air Port Express with the Spitfire 24 Bit DAC from Firestone Audio. They can be had online for around $250. This made as wonderful improvement over the AE's analog output.

I've been using the original X-DAC from Musical Fidelity (about $150) with my Mac Mini optical output. It was not that listenable until I added a jitter reduction unit from Theta ($125). The results are now very listenable. Read the threads and you may find other options.