Can’t apply Pricing Rule for Transaction

Hi there! I’ve tried to apply pricing rule for transactions and it just doesn’t work. It applies to Items, Item Groups and etc but not to transactions. Can you help me with that? How it supposed to work?
I need to add margin to every order customers depending on the cost of this order. I have the latest release of ERPNext v12

Perhaps I should explain what I want to achieve.
Item Prices contain prices for which I get products from my Supplier. Margin for every order always different because it is been calculated dynamically. For example, if combined cost of the order is >$50 then margin would be 10%. Or if >$50 and <$100 then it would be 13%.
I did some research and found out that Quotation (or Sales Order—whatever) doesn’t have Global Margin field. Also saw variant with negative discount Pricing Rule, but I want to be able to set discounts.
I’m thinking about implementation with Tax Rule but then reports will be ruined.

Do you have in mind the best way to do this?