It would depend on the size of the swamp cooler and the local electricity rates. In general, though, it would cost about 10-12 cents per hour to operate a medium-sized swamp cooler.

A swamp cooler can be had for as little as $150, and it's a good bet that it would only use between $5 and $10 of electricity per day to run. So the total daily cost of running a swamp cooler would be somewhere in the range of $5 to $15.