When figuring depreciation of my home for my home business, do I use the cost of my home I paid 20yrs ago or the current value of my home?

You use the LOWER of:

• Your Adjusted Basis (usually you original purchase price, plus cost of improvements, minus any prior depreciation, or
• The Fair Market Value when you started using it for business.

TurboTax should actually ask you for both of those values, then the program will choose the one to use for the basis according to the tax laws. 

The information provided by AmeliesUncle explains which value the program will choose to use.

@jacquedodds