The reason mom and pop stores are more money for the same item as a big box store is because of volume. Walmart is the largest retailer in the world plain and simple. They tell the manufacturer what they are willing to pay for an item and the manufacturer can either agree to sell their product at the price Walmart is willing to pay or they can say no and miss out. Mom and Pop stores can not compete with this. Walmart might spend say $5,000,000 on inventory in a day while a mom and pop would spend that in a year or two years.
There is one large problem with the loss of other retailers. Once there is only one place you can shop and there is no one left to compete with the store can then raise their prices to whatever they want to and there is little anyone can do about it since there is no where else to shop. Wlamart's business practices are dirty and yes they may employ many 1000s of people, those people are paid a low wage, are often not given enough hours to maintain their benefits, and are often treated poorly. My younger brother worked for them while attending college and told me first hand how poorly the employees are treated.
Another example about how we take it in the shorts from large corporations. Look at the oil industry. The large corporations are buying up all the smaller companies. Now that there are very few small companies out there the larger companies can now set their price as high as they want. They can say the reason for the price hike was because of having to shut down a plant for maintaince or because the middle east or what have you but the facts are there are surplus' in place for these very reasons.
When you have many companies out there competing against one another they have to keep the price low. When you have one or two companies out there they can do what they want.