Insight's more about understanding the motives/emotions of creatures than assessing places or things, but I agree that Investigation would be a good skill for all-purpose appraisal.
I also agree that anything like this should be passive, but a problem that occurs to me regarding my previous idea of using proficiency bonuses is that strictly limiting the scaling of any savings to level progression could easily make it start and end fairly insignificant since the way these games go, you often have a surplus of gold later on and will be less likely to have a need or appreciation for the increased benefit as a result. On the other hand, basing the extent of the savings on the associated ability modifier could leave us with a situation where the effect's too static.
I was dumbly thinking of insight where you judge someone else's appraisal, woops.
Depends on how we have to use gold, even in EA you can end up with more gold than you could think to do with cause Astarion is pretty good at pickpocketing.
