Nah, at most it would make sense to cost materials like Hardwood and Iron and other materials. But not money. If it would cost money, it would also make sense to force your prisoners to build them, to avoid paying troops more. Would be cool if you added hardwood from inventory. So yeah, I'd like using materials if possible to build siege weapons (wood, iron etc). And also limited ammunition for ballistas as well would be awesome... and make them a little bit stronger. Maybe buy ballista bolts and use them if you are going to build ballistas in a siege, and also buy rocks (heavy), used for catapults. Or, prolong your siege preparation time and make troops forage/mine/cut wood for the materials if you don't have certain materials and you want them.