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Is there a “standard deduction” for Line 5 on Schedule A of Federal taxes?
The $10,400 is in the question, in two pieces.  His employer withheld $8000, and her employer withheld $2400.  Thus they paid together $10,400 in income taxes, which are deductible if you itemize deductions and choose income taxes over sales taxes (you can deduct one or the other).  There's nothing "standard" about the amount, though it is standard to take the income tax deduction (almost always higher than sales tax).