The reason why coffee blends are cheaper is because coffee blends incorporate a mix of both high quality coffee beans and also lower cost coffee beans.
By incorporating and using both high quality and lower cost coffee beans, it allows the coffee roasters to get a desired flavor profile while also keeping the roasting and production costs of the coffee lower, because the coffee roasters are not relying solely on expensive and high scoring coffee beans.
Also coffee blends are frequently produced in larger quantities than non coffee blends, which also further contributes to the lower price point of coffee blends.
Roasters of coffee blends can also use a smaller proportion of expensive and high quality coffee beans and then use them with a larger quantity of more affordable and cheaper coffee beans to create the desired flavor.
This then reduces the overall cost of production of the coffee.
Blending of the coffee beans also allows the coffee roasters to maintain a consistent flavor profile by using different coffee beans, when certain coffee beans are scarce or expensive due to factors such as market demand or weather.
Also coffee blends are also often produced in larger quantities than single origin coffees are, which can also result in economies of scale and lower prices for the coffee blends.
Coffee blends are also often created to target a specific flavor profile, and roasters may even prioritize sourcing of coffee beans which can consistently deliver that flavor, instead of focusing on the highest rated coffee beans.
And single origin coffees, especially the coffees from smaller farms or microlots, often will have higher costs for production and are also more expensive.