Hi,
Well, if the files are being moved and not copied then:
- In the source Site Collections in the Retention Document Library you will have copies of your files since the retention policiy is doing its job.
- In the destination site you will have the files you are moving.
So in summary, it's not a copy then delete thing because the retention policies

In the question I stated "move files from one library to another in the same site collection" so your answer is not to this question.
Will this move cost double the storage or not?

I tested a move with the SharePoint UI and did not see any file added to the retention library.
So, I assume I will be able to do it programmatically.
Thank you