i would make a "normal add on" (in your terms) and just include also dlc-content. and for that package i would charge <price of add on> + <price of all dlcs (maybe a small discount>. and for sure nobody would notice because nobody knew that i put dlc-content into the add-on and nobody knew what the "only-add-on-price" would have been.
Why should you think in terms of DLC-content to integrate when you make an expansion, unless you created small content you don't want to integrate via patches for free?
I mean: Why imaginary separating the whole into DLC-pieces and calculating the price based on possible prices for parts of your expansion's content? That's a very capitalistic logic, and Larian Studios is a good step away from that (which does not mean they don't calculate).
DLC and Expansion pack, nowadays, are synonyms. After all, you don't really believe that an expansion pack would be marketed as such on Steam? It would be in the DLC tab.
Expansion Packs are Downloadable Content but thinking about DLC most people think at smaller content packs. Like the original Wolfgraff and Bairdotr DLC. (It was Free, so no bitching about that.) Or the typical "Get one new Horse Armor for just 4,99€!" DLC.
An Expansion massively expands the game with new Areas and normally a new chapter of the storyline. It's not just random piece and bits but an actual playable hour-long new adventure with new enemies, content and probably improved game mechanics. Think about the bigger Skyrim, Age of Wonders 3 or ARMA 3 DLC's. We should clearly sepperate that from the normal DLC and just call it expansions.
