Is the cost basis of sold iso stock that was exercised in 2000 and sold in 2018, the original grant price of the exercised price ?

[TomYoung]

In this situation your basis per share is the per share exercise price.

Tom Young

[KarenJ]

Yes, it is the original grant price.

Please see the TurboTax ISO FAQ attached (Section #5).  It is quite helpful.

https://turbotax.intuit.com/tax-tips/investments-and-taxes/incentive-stock-options/L4azWgfwy