I believe this would be the best route.
ATH A500 do not need to be amp, which I think the dt 770's do. But thats IMO.
Get a Chaintech AV-710(soundcard), new or used, should be under 25.
Best price for a new A500 would be audiocubes:
AudioCubes.com - Audio-Technica ATH-A500 Art Headphones - ATH-A500 -
I might be selling mine for 70 shipped. But really Ive had this headphone for almost a year, and it sounds great, gaming is fun (played 1.6 for about 2 years) never let me down. Really comfortable headphone.
Anyways if you go new for both prolly be around 120$, if you can find any used should be under 100. Goodluck