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> There are no fees on Polymarket

"Currently, small fees apply to Crypto and Sports markets.

Starting March 30, 2026, this will expand to include other categories like Finance, Politics, Economics, Culture, Weather, and Tech" [1].

More critically, Polymarket doesn't pay interest on deposits. (Kalshi does.)

[1] https://help.polymarket.com/en/articles/13364478-trading-fee...

Thanks for the correction! I have to admit it was a while since I last looked into this and I shouldn't have been so confident.

There is a fee implicit in the market spread. It's formed out of the time value of money w.r.t. the cost of NOT trading as well as the adverse selection faced by those with standing offers.

Increased insider trading will increase spreads.

Then explain why the average prediction market has a smaller spread than the average equity option market.

Depends on the event:

https://docs.polymarket.com/trading/fees