A 4-year max extension offered summer 2017 to take effect summer 2018 would start at 25% of the 2018 cap which is projected at 108M, and would have 7.5% escalators each year. So that would be 27M/29M/31M/33M, or a total of $120M over 4 years. For reference, CJ's extension that takes effect this summer is $106M over 4 years.
Of course, we don't know if Nurkic and his agent have an amount lower than that that they would accept as a guaranteed payday rather than gambling on another year of health and performance. For example, 4y/100M would start at 22.5M in year 1. If he keeps up for the remainder of the season what he's been doing so far, he's easily worth that much.