As other posters before me have already laid out, depends on what the PC is gonna be used for mainly.
For a general purpose one, it's always the monitor $100-200. (Roughly - CPU-$80-120, Mobo $60-100, RAM- $20-50, HDD- $40-60, PSU- $30-50, Case- $20-80, GPU- $30-100).
For a PC that'll do something graphics intensive though, it'll be the Graphics card. As for running multi threaded CPU intensive apps, they'll need a much more powerful processor.
And these are for consumer end desktops only. High end server processors alone can cost a whole lot more than a whole mid range quality PC in it's entirety.