My husband (US resident) and I (US citizen) want to transfer $50,000 dollars from our bank in Mexico to our bank account in the US. This money is from a business investment that we made 8 years ago in Mexico and has already been taxed in Mexico. Do we have to pay taxes on it again here in the US?

There is no tax on transfer of your money to the U.S.

You are required to report your Mexico income on the U.S. tax return. You must report your worldwide income on the tax return.

Another filing requirement is Form TD F 90-22.1. Your U.S. Tax Return: U.S. Citizen or Resident with Foreign Income

You are in fact required to report the income as was stated on your US tax return, in the year earned. For example, if you earned it several years ago, you report it in that year only. IF you did not report it at all, you need to amend your tax return to include it, using form 1040X.
However, you do not have to pay taxes on it again. You do this by taking the "Foreign Tax Credit" on your US tax return for that year. Complete form F-1116 to calculate this credit. You can get more information about foreign tax credit in Publication 514, available on the IRS website.

Remember, the US federal income tax, is a tax on income, not a tax on transferring money. However, if you do move $10k or greater at any one time in or out of the US, you have to complete an informational return. Form FinCen 105. This form is not required to be completed, however if you move the money using normal banking procedures, i.e. bank to bank transfers. See the instructions for this form for more information.