I will have to think longer for a USD250 or less solution.
I can very quickly recommend the new iFi xDSD and Pro-Ject S2 Pre Box Digital DAC - each at USD399, if someones budget were to stretch a little more. Both sound superb via optical input.
From personal experience only (not expertise), when you start to go lower in price than the above two, the Chromecast’s built in DAC looks to be a good enough solution. Just get a 3.5mm to RCA cable and hook-up to your amp.
http://www.belkin.com/au/p/P-AV10092/
But I’ll have a longer think of any <USD250 DACs with TOSlink input.