price might be rocketed because it's 4k built, costume must be in 4k high detail.
i'd say $4.99 per piece
"4K high detail"... XD
PS4 and Xbox One are not even able to render anything near 4K nor have they enough RAM to use so called "4K assets". That's just empty marketing talk anyway.
That reminds me of the "Soft Engine". Simple breast bounce enhancements and adding a subsurface scattering effect (which nearly all modern games use to enhance skin) was talked up to be this proper game engine. So many actually believed that, and now they are making a huge fuss because it was apparently ditched for the Dynasty Warriors 9 engine, which actually is a proper game engine. Don't worry, subsurface scattering will still be a feature of the DW9 engine as well as breast animations. It remains to be seen though how pronounced they want the animations to be.
Back on topic, characters and costumes will still be made of the same old polygons and textures, nothing 4K or revolutionary about them. There are few PC games that indeed use higher quality textures than the console versions, but usually models and textures look significantly better on PC not because they are actually of better asset quality (or "4K quality"), but simply because they are displayed in higher resolution. Higher pixel count, less aliasing and better texture filtering automatically make models look better. But that's not due to more work and effort of the game designers, it's just a positive side effect.
That being said, I expect the DLC costumes to be of higher quality, especially in the texture department, than DOA5 and I also think they will raise the price. More than double the current price I don't think is reasonable though and not even in Koei Tecmo's own interest. If the costume prices jump that high people will buy way less costumes, which will hurt KT in the long run. Just image how much a costume set or even a season pass would cost. 50$ for a set and up to 200$ for a season pass would deter so many customers.