You will need to come up with a number the best you can.
Year-end balance from 2016 is not correct.
Back in the year 2000, the max amount you could put in was $2,000. So try to reconstruct how many years you may have contributed and multiply that by $2,000.
Then put this information in your tax file to show how you arrived at this number.
The attached link is a pdf for the year 2000 that shows the max amount for that year. https://www.irs.gov/pub/irs-prior/p590--2000.pdf
IRS will generally accept a reasonable method used to determine this cost basis.
