In my experience, it's rarely cheaper to build your own than to buy one off the shelf, or second hand if you don't want the latest specs, but building my own is the only way I would go. I have built three now, for friends and family. Not made a penny on any of them for myself, but I did have the enjoyment of shopping for the bits, putting them together, and the satisfaction of knowing it was all my own work.
You might like to visit
this thread, which is quite an entertaining read.