Cost should simply scale with the corp size but in reverse of what it used to be. Instead of making it more expensive to declare war against biggest groups, it should be the cheapest to declare war against them. It makes no sense whatsoever that it used to cost 500M to declare war against a 10k alliance (which inherently has the means to defend itself) but only 10M against a corp with 10 characters that realistically has no way to defend itself against a war dec group.
To fix perceived issues with wars, it should be more expensive to declare war against small groups than big groups. It is entirely reasonable to make a war dec against a 1 man alt structure holding corp cost 500M to require the attackers to have a goal and not just yolo wars. And it should only cost 10M to declare war against the hugest alliances so that they are not safe from attacks and every one can try and farm them.
Furthermore, I do not believe that alliances that are bigger than 5000 characters should be war dec immune. (Personally, I think even 1k strong groups should not be immune but I want to be reasonable). Social corps can be a thing but the number of chars in such a corp should be limited and not a Get-away-free-from-wars card. If you have nearly 35k chars under your banner, you are not a social corp any more. You are a power house and should act and be treated like one.