Putting a gold requirement on long rest would deter using long rest at all. If you put a fatigue and gold requirement on long rest along with weight for such a item, this would become tedious micromanagement. Along with problematic with requiring X gold per day (along with gold requirements already in the game).
There is already sign of long rest requirement via dialog that characters say out of no where. This leads to possibility of a day night cycle of some sort, I would guess a fatigue system - one that reduces attributes or combat effectiveness. Along with locking long rests to the feature, so it would be time, combat, or checkpoint triggered.
Compiling more ontop of it will lead to a survival game (starvation, camp equipment breaking/repair, exhaustion, etc).