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.
Here everything is assembled to check if it's okay...
Now all the boards were painted with left over paint that we had around the house...hence the colors!
before adding the front wheel ...first axle ...then the pay loader arm then the wheels. After adding the wheels tape the ends of the axle with thick tape so the wheels don't come out. Cut out circles and rectangles for the front and back lights...paint them and stick them with white glue/ hot glue. Stick the hood to the top of the body too.
Well that's about it...it cost us nothing as we had the paint and washers and string!