I built one years ago, worked great. If you have a indoor rock climbing gym near where you live, ask them if they will let you have some old climbing rope. Most gyms replace the ropes once every month or two so they have lots laying around. It is a little bit of a liability for them to give it to you, but just insist that you will not use it for climbing. Ask them to show you how to butterfly coil a rope. Take 2 coiled up ropes about the length of a human spine and coat that with duct tape. Then wrap ropes around this to build up desired torso size. Leave about 4-5" out of top for the neck. Find some good tuff foam to cover the torso with and finish with tape. Wrap rope into a tight ball for head and cover with tape. leave some rope visible from tape to tie it to the spine. Use the rope to make some arms and legs and tie them to the torso. This isn't very pretty, but very cheap. If your good at anatomy use can use additional strads of rope and tape to make properly placed tendons for lifelike movability of joints. Climbing rope is pretty heavy mine weight in at about 135 lbs.
I just tore mine appart after it spent 2 years out behind the garage, The tape was a shot but the main body was still good.