You original cost basis is the fair market value of the inherited property on the date the relative passed, plus the amount you paid for the share you purchased. So, using your figures it is the $50K your wife inherited, plus the $50K you paid for the other share, for a total of $100K. To that you would add any improvements to the property since you acquired it.
As you noted, any depreciation "allowed or allowable" on the property is deducted in the process of calculating the gain on the sale, and the "recaptured" depreciation amount is taxed at ordinary income rates instead of capital gains rates.
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