Step 1: The farmer's share in the consumer's rupee depends on how long the marketing channel is and how much processing the raw product goes through before it reaches the buyer.
Step 2: Rice, milk and gram travel through fairly short channels with limited processing, so the producer keeps a large part of the price paid by the household.
Step 3: Cotton is different. It passes through ginning, spinning, weaving, dyeing and garment making before it becomes a piece of cloth. Every one of these stages adds a margin that never reaches the farmer, so the raw cotton grower ends up with the smallest share of the consumer's rupee among the four.