For me it cost was zero!! my son loved it and ...come to think of it,,,,it can be used for a long time to play with after the purpose of costume is done!
As you can see from the images....it's all cardboard boxes!! For the wheels i attached 2 cardboard boxes against the grain...then taped the edges so i gave a nice finish. For the axle i got long cardboard tubes (you can find it in material shops! they will give it to you free once the length of material is over!). The lifting up arms ( i don't know what it's called)....well...like the tiers i pasted 2 boards up and cut the shape out....later i reinforced the arms with plastic sticks and taped it up...i also got a light Bristol board and hot glued it to the arms.
For the body as you can see form the pictures....i cut out the windows....(later i added another box under it as it wasn't high enough for my son to stand inside it...you can see it in the second picture). For the bottom of the body i removed the flaps and cut out a square smaller than the top box opening so i can be hot glued and taped properly. As you can see all the edges and joints were reinforced with tape.