If I check the Larian Vault DKS is €40, while FoV is €20. You can always email support@larian.com and request they reevaluate FoV's price point (it wouldn't hurt to try, anyway).
Many digital distributors elected to only carry DKS (Steam, at least, had a discount for people who bought D2:ED through them when DKS was first released).
DKS occasionally goes on sale at Steam, Amazon download, etc, which should eliminate the feeling of re-buying D2:ED, if it is less than what you figure just the add-on should be.
Xbox players didn't get the option of just buying FoV, since it was too big for digital download and Larian couldn't get a publisher for a retail release (only the German version of FoV got a DVD release). FoV can only be played on Xbox as part of DKS (starting a new character to play through the remastered D2:ED or a level 35 character at the beginning of FoV).
Re-release collections are pretty much always cheaper than the original plus add-on(s); the same thing can happen with remastered or anniversary editions of movies, etc. I bought the platinum edition of NWN (with 2 add-ons) when it was the same price as the gold edition (with just the first add-on), and then they came out with another add-on, and then a diamond edition of the game with all 3 add-ons.
