you could heavily plant the pond and let the plants filter the water, assuming your fish don't eat the plants.
Also I suppose you could just throw the filter media on the ponds bottom but thats not anywhere as effective as pushing water past it.
the plant scenario is probably the best answer, but why don't you get your electric bill and do the math to figure out how much a pump will actually cost in electricity.
it will probably be around 6 dollars a month.