No need to worry about the cost, you can be sure Blizzard will worry about that
We don't have the data to determine (nor would our conclusion ever matter) if X or Y ways or doing are cost effective and/or how profitable each is, let Blizzard do that.
You can keep claiming it's not cost effective, it's not profitable, or it's not profitable enough, but you have about as much proof as anyone who says it is profitable. Educated or not, it's just guessing and predicting based on loose information. Keep at it all you want, but it's just prolonging the discussion with no true bearings other than extending the post count with bickering over "It's profitable! No it's not! Yes it is! It isn't, look at this logic I made! I call BS! etc".
All we should do honestly is discuss imho is things like:
- What the interest in Legacy realms can say about the current state of the game;
-- What exactly are the main reasons some players prefer legacy states of the game over the current;
- If and why you would play or not on a legacy realm;
- - If not, would you be ok with them being added either way?
- What kind of legacy realms could work;
- What kind of legacy realms would be more popular;
- How to market legacy realms disticntly as to not give misleading advertising to the main game;
- What alternatives are there to legacy realms that can satisfy this niche;
Really, put yourself in Blizzard's shoes. Would a bunch of players discussing how cost-effective adding somethign to your game do you any good? Even if they somehow were able to actually draw in to reasonable logic conclusions, wouldn't you still do your own internal investigations / analysis to know for sure?
Stuff like the above, however, can help them determine wether it's worth it or not to make legacy realms - or something else to please the legacy realm niche - And have a better idea of what exactly the players want from it, be it to know how exactly to go about it, or even just to keep in mind when developing new content even if not implementing legacy realms.