I'd look for 2nd hand as well. We bought DS a starter bin of just bricks, and then once we knew he liked it, MIL mailed down DH's childhood Lego collection (all mixed together at that point but I was totally fine with that). They look as good as new.
Those things are so well made that if you can find them 2nd hand for a decent price, it is a great option.
We took DS to a Lego store a few months ago and he had fun filling one of their cups for 10 or 15 bucks and put a lot of fun, novel pieces in there to build with.
Mama to DS-2004
DD-2006
and a new addition-ds born march 2010