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).
