Well, if you go out side of your tank, you will have to have at least one pump to return the water or to pump to the filter. You can use gravity to push the water in the other direction. Small pumps can be had for as little as ten bucks and sometimes five when they are on sale (Harbor Freight). Nothing says that you have to have brand new stuff either. Keep an eye on the classified ads, garage sales, Goodwill, and Craigslist.com.
What size tank do you have and what equipment do you have right now? An undergravel filter powered with an air pump is probably the cheapest and most reliable off the shelf system you can get.