Cost of Your Home in "tell us about your home" section

[kare2k13]

In the deductions section for home office, in the "tell us about your home section", what value is it looking for in the 'cost' section?  Our home was purchased in 1987, and we started using a portion of it for the home office in 2017.  In the "Learn More" link it wants the "lesser of: the item's fair market value or the basis in the asset."  The "fair market value" is considerably more than the purchase price, so what the heck do we enter here???  VERY confusing and unclear.  Thanks.

[KristinaK]

You enter the "lesser of". If the fair market value on the day you placed it in service was higher than your basis (purchase price, plus improvements, less possible depreciation), then you enter the basis. 

[kare2k13]

Thank you SO much! 

Is there any way you could help with my "mortgage interest" question?  I had only one reply which didn't help.  Thanks.