Also, put another way, those EU countries are probably getting charged less than the average costs to actually make the doses themselves. That's fine.
An issue is that R&D is a fixed cost rather than a variable cost, so determining how the R&D costs are to be split among AZ's customers is pretty hard and potentially prone to abuse. Are they to be split among all expected customers before the pandemic ends? All expected customers during the lifetime of the vaccine? During the lifetime of this family of vaccines?
Given that apparently it makes up the majority of the price, it's quite an important detail.