About £200 maximum. I was thinking about a Cambridge Audio DACmagic, but I have seen a few really cheep DAC's on Ebay and Amazon for under £30 and wondered how good they were?
These cheap DAC's will just be a waste of money. They should work fine, in and of itself, there's nothing "wrong" with them, but they won't do anything more than the existing DAC that the Apple TV already has... so they'll be redundant. Spend a bit more on a proper DAC and you should get an actual upgrade to your sound quality.