cash in on

To cash in on something is to benefit or make money from an event or situation, often in an unfair way.
Examples of use:
a) BBC News headline: Israelis cash in on Kosher wine trade.
b) Some Londoners cashed in on the Royal Wedding by renting out their homes.
c) Times Online headline: Investors cash in on BP crisis.
d) Airline companies are cashing in on the demand for cheap flights.