[unholiness]

This is true about arbitraging between markets on PredictIt, but I do think the impact of fees on market prices (and on ability to make money on PredictIt) was usually overstated.

PredictIt has two fees: A 10% fee on winnings, and a 5% fee on withdrawals.

The 10% fee on winnings is only on winnings. So when buying a share for 90¢, your $1 won will earn 99¢ and pay 1¢ in fees.

This means in a market with multiple candidates, betting "No" on all of N candidates will always earn you $(N - 1) without fees. That's is a winning bet iff all the "No" shares add up to $(N - 1.1111). In practice, the bias toward "Yes" shares alone is strong enough that buying all "No" shares, even at current asking prices, usually cost between $1.07 and $1.10 less than $N.

So, if you're only buying "No" shares in markets with multiple outcomes, the prices are set close to a point that entirely negates that 10% fee.

And of course the 5% fee on withdrawals is only on withdrawals. If you send the money straight back into another trade over and over, the fee is diluted enough to be irrelevant to any individual trading decisions.

[LittlePeter]

I don't understand where N-1.1111 comes from. Isn't it enough for all the shares to add up to less than N-1 for there to be a profitable bet?

If all no shares add up to S < N-1, then gross winnings will be N-1-S > 0, which when multiplied by 0.9 to get net winnings will still be positive.

[unholiness]

From the fees. 90% of a $1.1111 profit is $1.

Notably, you don't need to post $N to make this bet. PredictIt calculates your worst case scenario and holds only that money. A set of "No" shares worth N-1.12, if it existed, would be literally free to hold. Fees are included in that calculation, but also fees are based on the difference between that cost to hold and the return. So yes, profits mean different things in different parts of that calculation in this degenerate case.

A full set of Nos worth N-1 would be a guaranteed loss of $0.10 per share from fees. PredictIt would hold that 10¢ and you would never see it again.