Ive always thought of it as what is thr best resource to stock. Your choices are food, tech, cash, or units. They all have a cost to stock. My guess is cash will be the best option this set.

But it is best to keep growing right now.

Quick calculation.

Jets cost .14 + .001*70*1.1 = .217 per turn. If you sell the jet for 110 and stock the cash, the cash will cost you 110*.9*.001 =.1 per turn. If your last turn produces 1NW/160 cost or 1 jet for $96, you have to have 820 turns of holding cash rather than the jet to recoup the NW. So that would be your trigger point to stop selling.

I know I didnt explain that too well. Sorry :) it is complicated and I dont have much experience muself.