I don't believe a monitor fitting your criteria exists.
For graphic design you want h-ips not e-ips. Those start around $500, the best (imho) is the u2410 fir $550 but that's 2 years old.
The other screen I'd look at is the Asus pro-art PA246Q for $500.
The panel you really want is the NEC 2490 but thats $750.
Less than that and you're looking at e-ips which isn't true 8bit per channel.