Do the materials cost him $20? According to the comments, he is using honeycomb cardboard, like from HONEYCOMB BOARD| CORRUGATED PAD | CORRUGATED SHEET | CORRUGATED CARDBOARD | COROPLAST | CORRUGATED ROLL
. I looked around a little and that was the best price that I saw, $18.75 for one 4'x8' sheet, with a discount for bulk orders. Amazon shows timing belts for just over a dollar and then there is the cost of paint and other chemicals, the tires, and what he charges for his time, currently three hours.
I am afraid that he would be selling them for more than that.