The reason why Walmart's rotisserie chickens are so cheap is because Walmart's rotisserie chickens are sold to customers as a bulk leader, which is to attract customers that are likely to buy other items after they purchase the rotisserie chicken.
Other reasons why Walmart's rotisserie chickens are so cheap is because they are made in bulk from large and discounted purchases, and Walmart also uses chickens which are nearing their expiration date for the rotisserie chicken.
Selling their rotisserie chickens below Walmart's cost is a marketing tactic to bring people into Walmart.
And the goal is to attract customers and then have those customers purchase additional higher priced merchandise.
Walmart also buys their chickens in very large quantities, which results in them paying a lower per unit cost for the chickens that can then be passed down on to the customer.
And chickens that are close to their sell by date are also used for the rotisserie chicken at Walmart, which turns a potential loss into a small profit and extends the shelf life of the chicken.
The Walmart stores also have established infrastructure, such as the ovens and staff being already in place.
This allows Walmart to produce the rotisserie chickens at lower cost than a home cook could, especially when you consider the time and effort that is involved in making the rotisserie chicken.
A saltwater based brine is also often used in Walmart's rotisserie chickens that is injected into the chicken to improve it's moisture and flavor, which also helps in using the chickens that may otherwise be discarded.
In short Walmart uses rotisserie chickens are used to get rid of raw chickens that have not yet sold to avoid having to throw them out before they expire and go bad.
And this helps to lower the cost of the rotisserie chickens and makes them so cheap to buy.