What someone sells something for depends on their costs plus something for profit. However, some of the big box stores might sell a product as a loss leader to lure you in.

I would buy what is cheapest in your area, even if it is straw, pine straw or chipped bark/wood. My area has Cypress wood chopped/mulch. I am far from the Southern area where this tree is harvested for many uses, but it is sold cheaply chopped for mulch at my local HOME DEPOT. It is fairly regularly marked down per bag of mulch. I must admit to purchasing 5-6 bags of it. Maybe more than that at a time.

My latest visit to my local Home Depot showed mulch on sale, special price, for $2.50 per 2.0 cubic foot bag or $33.75 per cubic yard. A local landscape supplier is selling wood chip mulch for $17.95 per cubic yard, in 5 yard loads plus a $25.00 delivery fee which puts the price of that mulch at $22,95 per cubic yard, considerably less then Home Depots price.