Well that depends on the type of user you are. Enterprise level can reaches into the thousands easily. The most expensive processor I can think of is Intel Core i7-990x for around $1200 but most likely selling for $1000 these days. Enterprise level SSD will probably cost you $5000 for the most expensive one. Professional level graphic card can go up for $3000. You get my idea? If you're referring to the average system builder. The most expensive one is probably the graphic card since some of us can afford the 580 for around $450 I believe? We don't normally go over i7-2600k which is around $300.
Hey I was just curious on what people spend the most money. Personally for me it's Screens, with two Acer 22" at a total of $650, but after I get my 8800GTX it will be that. But this is when I purchased my computer.
I already had my monitors, but if not then it would be those. Obviously you can buy much cheaper monitors though, a FULL HD 22" LG in the UK can be bought for only about £100. Price obviously shoots up if you want multiple monitors or 3D
Obviously the results can vary considerably, depending on if one chooses a 30" Dell monitor costing $1400, or a $1000 990X cpu, or a $500 X58 'Kitchen Sink Killer' mainboard, or an $800 gpu such as the 6990/GTX590....