Taxes to be payed with the cash the player has accumulated.
The more cash the player, the more taxes to be payed.
That way newer players can compete better with the bigger ones.
Like in:
Amount of cash in billions equals the percentage of tax to pay,
so...
with 100 billion or more the player loses all the cash,
with 10 billion 10% is to be payed
with 1 billion 1% is to be payed
with less than that no taxes (or a symbolical tax) are to be payed - the player gets a tax refund instead, paid from the taxes of the big players

Off course, the big player would want to invest all their cash everytime just before it