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David VarghaTypically sell for less than the unqualified card one grade below it. Very few cards at the PSA 6 level will get "OC" for centering as the centering requirements get far less stringent at that level. A PSA 8 OC that is centered to meet the standards of an unqualified PSA 6 would sell for a Psa 6 price, with perhaps a small premium for the coner and surface condition.
However, centering and corners seem to be two of the greatest concerns of many collectors today, so a "MC" designation is almost the kiss of death. a PSA 6 MC would be very fortunate to attract PSA 4 pricing. Many collectors completely eschew buying cards with qualifiers to the point that you may make more money on such a card in unslabbed condition or re-slabbed at the lower grade it qualifies for as not having the OC, ST, etc. designation.