The 1820 mintage of 27.07g .903 silver 8 reales KM#111 at Mexico City is not listed in the 3rd Ed. 19th Century Krause. The value is listed at $20 VG, $35 F, $50 VF and $90 XF. Unc. examples are too rare to be valued in Krause. The obverse looks VGish to me, but from the picture I can't discern enough about the amount of wear in the coat of arms elements to pin the reverse down. It could be anything from VG to VF.
