If you just use the cost overrides, not the dynamic costs, you will find that after it has gone below the treshold where the costs stops it wont start again even if population rises above reinstate treshold.
Also costX still can't be set to 1 sometimes. I've been struggling with this here this morning and kept restarting the sim forcing it to 1 but it changed to a lower value despite that the population wasn't below the lower treshold when using dynamic costs.
It may be difficult to reproduce this though.