According to a economic study commissioned by Wal-mart. Wal-mart saves the average American family $2500/year, whether they shop at Wal-mart or not. They study, which has stood up to much scrutiny, concluded that Wal-mart has a downward impact on prices economy wide and lowers prices across the board. Wal-mart makes almost everything cheaper for everyone.
However, other studies have shown that Wal-mart has a similar lowering effect on salaries.
It's had a massive negative impact on manufacturers, suppliers and other retailers.
Let's say wal-mart buys a computer system to seel in their stores. They buy >1,000 at a time, direct from the manufacturer and distribute it using their own distribution network. Smart business move, you could ship laptops to stores in the same trucks as you ship tennis balls and groceries.
Now, let's say that I, as a small retailer of computers, cannot order direct from the manufacturer because I am only ordering 5 units at a time. I have to ring up my supplier in Vancouver, who rings up his supplier somewhere else who can get it from the manufacturer.
Many people are involved throughout my supply chain. There are employees at the suppliers office, the suppliers supplier's office, the couriers, etc... while this is an inefficient system, there's little choice but to use it.
Because of the wal-mart model (and other big-box retailers) I need to convince my supplier to drop the price on the item. My supplier then says "we can't, we have all these people to employ to get you the item, we must remain firm". Then many small stores begin to ask for the same deal. Suppliers, realizing that the wal-mart RETAIL price is LESS than a small store's WHOLESALE price, drop wages, shed employees, etc... to try and reduce the price. I still pay more than wal-mart retail but that's about as low as they can go.
Now, multiply this problem across all the smaller retailers and supply channels that are competing with the thousands of products available in a single wal-mart store and see how difficult it is to survive.