This is not easy to explain; I’ll do my best.
The value or price that is offered for land by any prospective buyer is based on the unearned income potential when it is developed to its highest and best use, plus the speculative value based on future expectations of greater income.
It is not related to the cost of holding the land idle. Idle / vacant land yields no income at all — has no potential. A land speculator either sells for the highest price offered, or ...