Aren't ready-made DACs like $200 each or so? I've built sound card DACs for about $40-50 in parts. Tack on another $50 for a portable computer like a Raspberry Pi + case.
Depends what you get - The FB3 with Quickshow from Pangolin is around $600.
An EtherDream DAC without any software is $200. Tack on $130 (Currently a sale on, so probably decently more expensive normally) for a copy of LSX to go with it.
Interesting, will a Raspberry Pi playback an 8 channel wave file?