What your mother paid for the house originally will be recorded at the county clerk's office. Usually it's filed with the original deed and construction permits and the such. SO if the recording office doesn't have what you need, the permitting office should.
If that house was at any time a rental, or if a home office was claimed any time during her ownership, then find the tax return with that info on it. It will have what you need for helping you establish the cost basis.
You can also check with the lender who provided the original construction loan and see if you can get the amount of that loan. The amount of that loan along with the appraised value of the land at the time of that loan could then be used to establish an original cost basis. I'm sure the bank required a land appraisal as part of that original loan, and that appraisal will be with the loan paperwork. I would expect the lender to change you a small "research fee" to go dig that information up for you. But you may be able to include those fees in your deductible sales expenses. (Not sure if you can or not really)
