It's easy. Just go to a number of websites and pick the cheapest! Right? Unfortunately it's not as simple as that. Here is a simple guide on what to factor in when comparing prices on the Internet and at your local mall.
Item Price This is the headline price that most people compare. It can actually change quite often as retailers (particularly the smaller ones) react to competitors' prices. There can be a huge range of prices depending on the sort of thing you are looking at. Try using one of the comparison websites (e.g. shopping.com, shopzilla.com).
Shipping and Handling This is the biggest source of variation and the thing that you need to compare directly to actually going the mall. You may offset a slightly cheaper price with a shipping charge and you will have to wait longer. Now this might be OK if you aren't in a rush, but it does need to be factored in as standard shipping can often take two weeks. Some companies make up the cheaper price with a profit on shipping and handling. I was looking for a grill recently and found one $9 less than the one on Amazon.com. However, the shipping and handling was $40 - $22 more!
Sales Tax This is the one people seem to think about the least. Tax laws are horribly complex, so I won't try to do a detailed explanation, but as a general rule, states require that sales tax be levied on the basis of the address the item is being delivered to and whether the merchant has a presence in that state (and what type of presence that is). So the national chains, those you might find in a mall, are required to charge sales tax on most items they sell. However, a number of internet businesses do not have presence in a lot of states and can therefore minimize the number of states they charge sales tax for, or not charge it at all! This can make a huge difference if you live in a state with a high sales tax.
Going to the Mall This is what you should use as a baseline. You can, of course, go all the way down to the store and find they are out of stock - something you find out up front using the Internet. If you are happy to pay a little more to avoid a trip to the mall then that is fine, however you shouldn't expect to always pay less and not have to drive to the mall.
So, in summary, work out what the real cost will be to you (item price, shipping, sales tax, cost of your time and gas) as well as factoring in the convenience and how long you're willing to wait.