I made a Lego Man Halloween costume for my fiance in 2010. It was very cheap, easy and everyone loved it! The hardest part was waiting for the paint to dry!
All I did was find a medium box that fit my fiance comfortably, plus a shoe box for the hat. I cut arm holes for him but it would be easier if I just skipped arm holes and let him put his arms over the box (hindsight!). For the pegs, I purchased a few packages of plastic food containers from the Dollar Tree (6 pegs for the body and 2 pegs for the hat).
I cut peg holes smaller than the widest part of the container so I could make it a snug fit (I also reinforced the containers with packing tape). I then took some wire (yarn would work well, too) through holes I punched in the box to make straps. Next, I spray painted the box and pegs evenly outdoors on a large piece of cardboard (wear a glove when spraying).
For the hat, I cut two holes in the shoebox for the pegs and used wire to go through those little holes on top of his baseball cap, then spray painted it. Let everything dry. For a completed look, I bought a $2 red craft shirt to match (long-sleeve would look better).
Voila! Lego Man is born!