Usually it's so much on parts, then something for assembly and installing the OS, drivers, etc.
You can't take just some percent above the parts cost since installed an OS can be a fixed rate. So for a 199 buck PC that could double yet a high end machine would be something like your calculation.
Does anyone have ideas about the bext value high end machines?
I'm seeing dual processor Xeon systems, ideally using 6-core Xeons.
Plenty of DDR3 - 48G and above options.
High spec MOBO
Disks, Video etc are secondary at this stage.
I looked at one supplier and when carefully examioned they were charging like 20% or 25% above the cost of the parts.