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PCA Mathewson black cap in SGC 20 recently went for $160 on ebay. An SGC 10 should go for less than that and, unfortunately, yours is missing a piece, which will bring the price down below a better condition 1. IMO, south of $100.
Other McGraws (different poses than yours) in 3/40, have recently brought between $110 and $145. I would think the Sovereign back would bring the high end of that range, maybe more if someone really wants the Sovereign back.
Hope that helps.