Earl Grey beat me to it; here's my review of Gevenalle when they started out as Retroshift. That might save you some $$ compared to parts including words like "Paul" and "Dura-Ace".

I agree with most others that you're not really doing an on-the-cheap build. Just yesterday I bought for my 14-yo son a 2010 Specialized Allez Sport (9sp tiagra/sora triple mix) for $400 off CL (and even that's above blue book, b/c of new tires).

But hey, the joy of working on bikes has its own value, which is more valuable for some than for others. What you really want to do is get your son in on the building, so he appreciates ("earns") his bike more, and will be better able and motivated to properly care for it in the future.