Discussion about value
While we're at this point, we should talk about value. Most AFOL's measure the relative value of a LEGO set by its piece count. That is the number of elements in the set. Clearly posted on the front of the package, a good rule of thumb is the set should cost about .10 cents per piece. Licensed Themes like Star Wars and Toy Story will command a premium price, presumably to help pay the licensing fees. So you can expect to pay more for those sets. But even that does not cover the whole story. The LEGO Group, which sells the sets does not value them based on pieces but rather by the amount of plastic used, most easily determined by weight. On the other hand, AFOL's are more interested in pieces because well larger pieces (SPUDS) are not as useful, so a focus on piece count serves them well, but for your little youngster, this is not the best measure. With that out of the way, let's move on.
So far, we have talked about standard online and brick-and-mortar stores; there is, however, 1 more. BrickLink.com is an online website that hosts many stores from the LEGO community, each of them run by their individual store owners. BrickLink is shinning if you are looking for specific LEGO elements or out of production sets or Mini Figs. There are no fees to sign up with BrickLink, only when you buy, and those fees are determined by the store you buy from.
Note: Since this article was published, Bricklink.com is now owned by The LEGO Group.