In Chinese discount language,
折 means '× 10%' — so
对折 ('paired discount') = 5
折 = 50% of the original price (i.e. half off). Other common discounts:
八折 = 80% of the price (20% off),
七五折 = 75% of the price (25% off). The 'fold paper in half' meaning is the literal source — the discount sense is metaphorical.
