Is it cheaper to install flooring yourself?

On materials alone, yes. In practice it depends on which floor and which room. A floating laminate or vinyl plank floor in a square, level room is a realistic project for a capable homeowner. Tile, hardwood, stairs, and anything requiring levelling, moisture management or waterproofing are not, and those are exactly where do-it-yourself jobs most often need redoing. Redoing costs more than doing it once. Worth checking too: many manufacturers require professional installation for the warranty to hold.