I will be building my own media rack.. However, when i build something i don't usually draw it up before hand... I would say 500$ sounds like a reasonable price. What materials did he use? That would be the big factor.. When i build mine, i will be using Rough cut cedar for the frame. and probably use Cedar deck boards for the shelves.
If he is using Yellow pine, he probably has 1-200$ in materials. However, his time is worth something, and the cost associated with the tools he has had to buy to build you something..
There are only 1 or 2 brands i would consider buying. However, i think building something will result in a MUCH better end product.
I should say that i am a big fan of DIY projects and as much as i can on my own; this is the perspective i am viewing this subject with.