Can someone help me?

I'd like to create a mod to change the prices of mini/scrap copters at bandit dynamically. 

I've provided a thorough explanation of this in several places, but the short answer is that it's not possible because the dialogue is duplicated client side.

The best you can do with the native experience is to trick the client into believing it has the full scrap amount and also use a plugin to enforce the different prices server side. The issue with this is that the displayed prices will still be the vanilla amounts which may confuse players.

The alternative to that is to make a custom UI. I may eventually get to this in some sort of custom vendor dialog plugin, but have other priorities for the next few months.

The Vehicle Vendor Options plugin also has an example of using the dialogue hooks to make the vehicles free by skipping the part where the player pays.

Thank you for you explanation. Much appreciated.