Head out to your local dollar store to pick up your materials. There you should be able to find some ladies pantyhose, a rubber inflated ball of a size of your choosing, some bungy cords and a hook you screw into your ceiling. These hooks are typically used for hanging lamps or planters.
Next go to the home improvement store to pick up a bag of concrete. Make sure to get a bag heavy enough that it wont slide when you hit the ball. 50 pounds should be enough and a bag shouldn't cost more than about $4.
First, cut a leg off of the pantyhose and slide the ball into it. Tie off the leg on both ends of the ball to hold it in place.
Next tie each end of the pantyhose to a hook on two different bungy cords. You can use pliers to close off the hook so that it doesn't slip off.
Next screw the hook into the ceiling where you want the bag. Make sure you leave enough room for it to rebound when you hit it.
Place the bag of concrete below the ceiling hook and loop the bottom bungy cord under it and close it off.
Finally stretch the top bungy cord up to the ceiling hook and close it with pliers. The cords should be taught so there is spring in the line. Give the ball a couple practice hits and make adjustments as necessary.
That is all you need to do to create a great double ended bag for cheap. Have a great workout!