Why?
 

Last year I bought my son (who is 10 now) a new Mongoose BMX bike that he uses for trail/dirt jumping. He also races MX. He wants a new bike and is thinking about building one (with my help of course). I have been looking at complete bikes as well as frames/parts. Why do I see alot of $300-$400 bikes that look descent but when am pricing frames I see most going for $200-$400? Do those complete bikes have that much cheaper frames/parts. It looks like building a bike is a much more expensive route to go but I think it would be good for father/son project. Thanks.

Yes, these completes do have way cheaper frames an parts. But theres more to it. For example a cheap 300 dollar-ish fit TRL 1 has a hi ten steel frame what looks like a sheet metal sprocket a blocky ugly cheap stem and pretty much crappy everything. The 600 dollar-ish trl 3 has nice tires a nice stem, is chromoly etc.. both are made in taiwan. A frame made by Fit would be around 300 dollars and would also be chromoly. It would also be made in the US, making it in theory, better. (Especially considering its made in the US by S&M)
You might want to be realistic with your son and set a budget. Quite possibly, within that budget, a complete might be more bike. 500 dollars buys a pretty sweet complete. 500 dollars could also buy you a frame and some wheels, maybe a sprocket too? I'd rather just have a stock bike than an incomplete custom!
Seriously I wouldn't build a custom for a ten year old unless I really thought he was gonna stick with it or unless I had enough stuff lying around to build one relatively on the cheap. (I'm sure by the time I have kids I will be building them sick custom bikes)

It has the potential to be a good and fun project. I helped a friend this spring with such a project for his two sons. Each built their own mountain bike. One of the kids really seemed to enjoy the building process. The other could take it or leave it. All-in-all, we did have several enjoyable evenings.

BMX bikes tend to be simpler than mountain bikes. That works in your favor. If you do build w/your son, keep the sessions short. I found that attention and enthusiasm seemed to drop after an hour and a half or so.

buy an ok complete then slowly upgrade it one part at a time
thats what i did :)
a lil over a year and $1,200 later i have a realy nice bike