You could ask a friend who's handy with wiring to rig something up for you kinda like this: You have a big container that's solid black, containing water. This sits outside somewhere where the sun will hit it all day long. There's a pipe running through it, that comes from your pond, and ends up in your pond. A thermostat is placed into the pond, and when the temperature falls below, say 76, it turns on a pump which pumps water from the pond, through the pipe which goes through this black container, and back into the pond, until it reaches, say 78 degrees, and then it turns the pump off. Since the container is black and in direct sunlight, it's temperature will get pretty high, meaning the water sitting in it gets pretty hot. The water from the pond and the water in this container don't actually ever mix, but the pipe sits in the hot water, and when the water from the pond is pumped through it, it heats up (good old second law of thermodynamics).
You might still need to run heaters at night, but this will cut your cost roughly in half.