Once materials are gathered you can make this in about ten minutes, and kids can help!
for contributed photos, more detailed instructions, variations on this theme, PVC resources, and links to other PVC mannequins.
Some of my Halloween work is also in other instructables:
Godzilla / T-Rex
General Grievous with Mechanical Arms
Step 1: Gather PVC Materials
# one bag of 1/2" slip elbows (not threaded) (~$3 for ten)
# one bag of 1/2" slip t-joints (not threaded) (~$2 for ten)
# a PVC tube cutter if you don't have one (~$8)
# PVC cement (~$4)
# optional: roll of black plastic contractors plastic to cover frame ($13)
# optional: polyester fiber fill (from a craft store) ($5)
Lots of costume components! Masks, capes, clothing, etc.
Step 2: Assemble
The base splays out in both directions to allow it to be weighted. I just used exercise weights or plates on the base.
The chest is made from three t-joints (joined with <2" pieces near the neck) and four elbows. If you don't glue the shoulders you can pose the arms.
Finally there's a view of the whole thing. By adjusting the body lengths you can make anything from a dwarf to a giant.