So a headphone amplifier with analog and digital inputs, and the digital via USB? Like this:
Or did you have something else in mind? The cheapest (commerical) balanced amps I'm aware of are the CEC HD53N (no digital in), and Schiit Mjolnir (no digital *or* SE in, and it's not released yet). You can add DACs to either. The Headroom BUDA (with the rest of it's stack) would also be comparable - more expensive than the Schiit stack; probably about at parity with the CEC setup (DA53N + HD53N).
You can alternately just get a free-standing DAC, and use whatever amplifier you like (sort of alluded to that above) - it'll give you more options and potentially save you money. For example the Cambridge DACMagic can drive XLR out (and costs about half what the DA53N does) and could serve as a source for the Schiit, CEC, or BUDA. Paired with the Schiit, you'd barely break $1k.
There's also been some DIY stuff with the Lovely Cube and ganged volume controls to create a balanced amp - no idea how complex or easy this is to implement, looks mechanically complicated though.