I just came across a very interesting study from DataCo, LLC, a provider of multi-faceted research and analytic services. They comparted WalMart.com with six other shopping sites, and actually found that at least two major online retailers [WalMart.com included] with the reputation of having the broadest selection and lowest prices fared the worst of all properties researched.

Here is more from their press release:

The consistency of the results provide a clear indication that to make the best and most informed purchase, a shopper's first stop when researching and buying online should be with comparison shopping engines rather than a specific retailer's site. ... The study pitted Walmart.com, Amazon, and five general-purpose comparison shopping engines -- NexTag, Pricegrabber, PriceRunner, Shopping.com and Shopzilla -- against each other to determine if any consistent patterns emerged that could help consumers navigate the Internet as efficiently as possible. Nearly 50 of the most popular products were researched, and the results showed a significant amount of pricing disparity on identical items between the seven providers. At the highest level, DataCo found that the comparison shopping engines outperformed Amazon and Walmart over 95% of the time on dozens of the most popular products online. In fact, of the products surveyed that are sold by Walmart, the giant retailer failed to provide the lowest price on any of them, and Amazon only provided the lowest price on two items.