What causes corruption expense and how to stop/lower it?

If you have over $2B cash onhand, you experience 0.1% loss of every dollar over 2B per turn played.

Food decays as well but I don't know that formula.

The classic way to get around it is to spend your cash on bushels, then "hide" those bushels on the market at a price you know won't sell, like $115 or something. That way you don't pay decay on the bushels while they're for sale.

When the end of the set, or a war, or some other reason to liquidate the bushels arises, you can sell them all on your private market and make big jumps in NW.

This process is called "stockpiling," and is how the top countries achieve their high NW end of round.

Now there is also a bonus points option to reduce your decay of food and money as well. It's useless in Express but can get to 100% in Primary, Alliance, and Cooperation if you focus on it.

I've noticed you countries in a few servers, do you play in any tags? I am in a war clan on Alliance (M4D), and a relaxed group of slackers on cooperation (NA). We'd love to have you in either, and you'd get to ask people who know the game inside and out about the game on Discord. PM me. :)

Bushels decay right from the first bushel at 0.1% per turn. But it rounds so you need to have 500 bushels to decay 1 bushel.

So if you have 1m bushels on hand you decay 1k bushels per turn.
And if you have 500-1499 bushels on hand you decay 1 bushel per turn.