rotisserie chickens are bred for that purpose and their lives are cut short to meet the cost and weight targets of the largest customers (walmart, etc), which means the facilities can produce 'more' in the same amount of time than roasters. they cost $6-10 here and $5-6 on sale (higher $ is at the regional convenience store chain, lower is wm), and i can frequently find 'old' ones in the cooler at wm marked down to $2 (yea, just two bucks each).

roasters are larger, priced by weight, and usually cost more (per bird) than rotisserie chicken. here, they're $10-12 at wm, $15 and up at the 'local' grocery store. they're rarely on sale.