home built PC market is probably larger than the entire Mac market. I don't have any figures to back this up but based on Windows' market share and my experience, 20% of PC's I've seen over the years are home built.
As (presumably) a home PC builder, 20% of the PC's you seen over the being home built doesn't imply much about the rest of the world...
The reason why there isn't much overlap is that workstations are often used in a professional setting by designers, where spending $3000 on a computer isn't considered a great deal. Home built PCs are rarer in this context.