Prices have been all over the shop lately. I really think it has to do with the willingness/lack thereof of sellers in the Us to ship to Asia.
Example, a nice pair of 604-8Gs. One pair, with XO, for $900, the other, with XO for $1700 in the last month. Difference was where the person was willing to ship.
Dunno about 811 v 511, i think it's just variations in demand.