About $20 per heater and a $20 Kill A Watt. You might find some cheaper heaters but make sure you do not get preset heaters.
You need to place the heaters in a high flow area of the reservoir as well. You can also get inline heaters like Skinnee uses in his testing setup, only he has several hundred dollars of equipment to go along with it to measure pressure drops, airflow, etc.
Perhaps cooling capacity could be worked out by figuring out time to take say 5 or more gallons of boiling water down to below ambient? I don't know the math - or how accurate such a test would be - but it is a lot cheaper than the electric heaters.
Maybe if Martin or Skinnee were curious one of those two would have the stuff needed to determine cooling capacity.