If you go to 500$ then i wont recomend you anything thats USB, simply its too slow.
Try to find one of the EMU 1616 or 1616M
it is probably the best on the market money-value.
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Cubase 8.5 Pro. Windows 7 X64. ASUS SaberTooth X99. Intel I7 5820K. ASUS GTX 960 Strix OC 2GB. 4x8 GB G.SKILL.
2 850 PRO 256GB SSDs. 1 850 EVO 1TB SSD. Acustica: Nebula Server 3 Ultimate, Murano, Magenta 3, Navy, Titanium.