Publicly traded shares can be tracked now more easily than ever before. We used to use a printed service to track historical stock prices and stock dividends/ stock splits. Now you can look at Yahoo Finance for historical stock pricing.
I took the liberty of trying it with Microsoft, and this is the link I used: http://finance.yahoo.com/q/hp?s=MSFT
Use that link, type in the symbol, and take an average of the stock price for the year. If it traded from $10 to $16 in 1985, I would use $13 as an estimate of share basis.
If dividends were reinvested, buying more shares, recent statements might mention those purchases. Stock splits or dividends will also be part of the Yahoo history pages.
In the event this is unwieldy or unsatisfactory, I typically estimate the shares were ____% of the sale price. Given the long holding period, I might go with 20-25% of the sale price as a cost estimate.
Thanks for asking at Just Answer. Positive feedback is appreciated. I'm PDtax.