Lawnmower blades are made from strips of 10B38 slit-edge steel. The bevelled cutting edge is milled in a machining center. Then they are austempered to anywhere from 37 to 52 Rc. Some blades are further enhanced with a carbon deposition process near the cutting edge.
"Cheap" is a relative term that takes quality into account. You can make the blades out of papier-mâché if you want.
Don
Kansas City