That is a difficult question and if you don't have the statements from back then even more difficult. You say Yahoo historical prices go back to 1980, so you have some prices to go back to. Do you know the original Ticker symbols of the fund through the years? Might be able to get some data using those???
Also, in the shares gifted to you, basis would be what the donor's basis was.
Did you try searching at for mutual funds at the SEC?
Here is a link to Pub 550. More in depth basis for mutual funds begins on page 43.
https://www.irs.gov/pub/irs-pdf/p550.pdf
One big problem is do you know how many shares were from re-invested income from the time you were gifted the shares to when you added to the fund?
If you can get say beginning, middle, and end of year prices for the funds in 1980 - 1987, you might use that to "average" the cost basis as a best estimate?
**Disclaimer: Effort has been made to offer correct information; but due to the discussion forum limitations, the poster disclaims any legal responsibility for the accuracy of the poster's response**
