Hedging against a big stock selloff can be expensive when using options. One way to reduce the cost is by using options spreads. A more complex spread that can be useful in this situation is a put butterfly. A strategist bought over 20,000 put butterflies in SPDR S&P 500 ETF (SPY) that expire at the end of November. The position can pay off if the market drops about 20%, which is why this is likely a cheap hedge against an unknown event.