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As non-residents of Mexico you may opt to use the residence section and permit deductions, but to be eligible you should appoint a tax domicile and representative in Mexico.
If you choose a flat tax on the sale of the real estate - it is 25% income tax on the gross income (sale price received).
if the normal (resident) tax is used, then there is about 40% rate income tax on the net income.
As you are US citizens - you should report and pay taxes on all your income worldwide.
Because the same income is taxable abroad - you may claim a credit for taxes paid abroad by using form 1116 - http://www.irs.gov/pub/irs-pdf/f1116.pdf
You do not need to have a corporation to use this credit.
As Mexico taxes are likely higher - there will not be any additional tax liability in the US, but you still need to report the income.