We bought a complete set of three year old World Book Encyclopedias for $75 at a library sale. Best money I ever spent as they've been used pretty much every day. My suggestion would be talk with your librarian and find out when they get rid of sets. Also, I'd check Craigslist and put a wanted listing. My opinion is that it is not particularly important that the encyclopedias be brand new. Maybe you don't want them to be fifty years old and smelly, but really if they are ten years old they are probably just as good for 99% of what your kid would want. If the shipping is reasonable Ebay may not be a bad option. Generally people are good about describing condition and you always have protection through ebay, paypal and your credit card.
http://cgi.ebay.com/2005-World-Book..._Nonfiction_Book&hash=item255e022675 $209, new in box, free shipping. This looks great!
http://cgi.ebay.com/World-Book-Ency..._Nonfiction_Book&hash=item3cb16bfb6f 2009, library set around $315, free shipping
