This will not work, goal of `buyCostItem` is only to calculate cost, noting else. even if I would add `getMonthlyBuyLimit` then this script hook could not still use it as it have only access to read-only game object.
To make it work we would need add `butItem` that will be allowed to alter game state but this is lot bigger feature to make it right.
I could consider adding this in some future when I will expand geosacpe scripts hooks more.