When Kalshi and Polymarket price the same event differently, the spread between them is a potential arbitrage opportunity — buy the cheaper side on one venue, the opposing side on the other, and collect the difference at resolution (net of fees and execution risk).
What to check
Are the two markets actually equivalent (same event, same resolution criteria)?
Is there enough depth to fill both legs at the quoted price?
Do fees and slippage still leave a profitable spread?