This is incorrect. You cannot exercise ISOs after 90 days post-employment. What you pay to the company at exercise isn't time dependent: the strike price is fixed at time of grant. What you may be referring to is the AMT, which does fluctuate depending on FMV at time of exercise, but that doesn't affect the 90-day limit.

Yea I wasn't completely clear I the description, not remembering exactly at the moment. Thanks for pointing out. I was referring to the option some employers will offer to convert ISOs to NSOs after 90 days. So yes you're technically not exercising ISOs after 90days. My point more being that there are options with some employers to not completely loose all rights to equity after 90days.