I did some quick calculations (Some of the values are off, and it really depends on how much land you are grabbing per day, and your total land goal). However I made a chart to show the best relative stopping points for at least what my current plan is. Basically the best time to switch from putting turns into cashing vs turns into building costs, is 3 days before you are finished grabbing. The break even point on turns vs building costs savings (purely on a cost point, turns if you are using for more CS to save you more turns later count for a few % more), is 50k acres.
If you saved all the extra turns you got, and used them after you finished building and grabbing, then the turnaround point of where building points is better is 135k acres.
https://imgur.com/jXD1E1I
The debate at this point would be, how many days ahead of schedule will you be with those extra turns. If we use the principal that an extra 80 turns is 1 day ahead, an extra 160 turns is two days, you would be able to add the daily incomes and compare it to the total building costs saved. That value breaks even at around 105k acres (1.25 days ahead of schedule worth of income and land).
I believe based on the above, that around 100k acres is the break even point typically, unless you are burning your turns for income purely, rather then something that boosts you forward (Readiness, grabbing, more CS), in which case the turns are for the most part wasted, as the investment return on them would not be high, then 50k acres would even work).