So make it like this. Set the base war fee to 50m. Scale it so that the fee increases with active player numbers in such a way that it works out more expensive to declare war the more people that are in your corp or alliance. Therefore the most efficient way to pay for wars would be using smaller player numbers in your war dec corp, not more. If they want to keep their war dec costs down, they should aim for less players in their corp, not more.
Example:
War dec fee 50m for 1-50 player corp/ally
War dec fee 125m for 51-100 player corp/ally
War dec fee 312m for 101-200 player corp/ally
War dec fee 780m for 201-400 player corp/ally
War dec fee 1950m for 401-800 player corp/ally
War dec fee 4875m for 801-1600 player corp/ally
You don’t even need exact tiers like this. The formula can calculate it based on exact player numbers because otherwise players will for example aim to have max 400 players to avoid the 1950m war tier.
Costs can be adjusted as necesary. Sure it mean an alliance of 1000 players will pay nearly 5 bill to declare war, but surely they can afford with those numbers as long as they’re not spamming many dozens of war decs.
You could also add something like, the first 3 war decs declared by an entity are charged at the base rate of 50m per dec. It’s only after you have 3 actively declared wars that the enhances fees based on player numbers kick in for additional decs. I don’t think it’s necessary to add this, but it’s an option.
The ever increasing costs for declaring war, at a rate set that increases the cost per player as player numbers increase, is to encourage people to work as smaller outfits, rather than bandwagon all under one alliance. They have the option for either way, but those who go smaller will be rewarded with effectively largely discounted war fees.
You’d also have to make it so the system cannot be abused, for example by declaring all your wars at the lowest rate on a new corp and then piling in all your members to the new corp. I gave an example of how to achieve this earler. The only problem was possible abuse to save war HQ. But if the war HQ idea was removed from the game, that would solve it.