Here is one excerpt from a lengthy and data-intensive post, which likely offers more than you ever would wish to know:
The internet can be blamed for the size and scope of the secondary LEGO market. On the website, BrickLink, you can find almost any set that LEGO has ever produced. In addition, the site keeps records of trends in the market and value of individual pieces. This site is invaluable to a LEGO collector and has given many the ability to grow their collections. Before the advent of this site and sites like eBay, collecting LEGO required going to garage sales. There are now whole sites dedicated to buying LEGO as an investment, but that is a topic for another article.
This creation and expansion of the secondary market in conjunction with LEGO now marketing some of their products to an older audience has made the prices of some old sets increase exponentially. On the extreme range, there is the UCS Millennium Falcon that is selling new for upwards of $2,000 (and close to $1,500 USED!). It sold for $500 new in 2007. Even non-licensed sets can run a premium, such as the Cafe Corner that was one of the original modular buildings. It was $150 new and now it can sell for over $1,000.
For pointers I thank Michael Rosenwald and Kevin Won.