I LOVED legos as a kid, and my son does too. Delaney (6) hasn't really gotten into them like Jonas did (12).
He's now into robotics, we did Lego League using Mindstorms. That was a great learning experience. especially the programming... he just recently built a programmable machine to shuffle a deck of cards for him... from scratch...
Sadly Legos are expensive! But there are deals like this to be had.
Also, check your local Craigslist, or even better, your local "young mom's community" - around here we have this group called Chesapeake Mommies that's a huge network of 30-something moms w/ little kids... we are always getting great deals on used stuff. And Legos are one of those things that are somewhat indestructible, "used" is just as good as new - as long as the kid wants to build their own shit.
We've probably saved hundreds of dollars buying Legos used, and I can't imagine the combined value of all of the plastic brick pieces in our house right now (especially those Mindstorm pieces)