Umm...When I bought it? 700$ built from boards.
Now? I think I've cycled through every single part in my PC except the processor and DVD Burner (Which just returned a CRC check error for the first time in 6 years, so it may be time ^.^)
But, My platform is still chugging along on my Trusty Core 2 Duo First-press ( My serial indicates it was in the first 100 batches of the processor) E6400. I float the speed between stock 2.13Ghz up to around 2.8Ghz as needed with my OCZ Vendetta 2 cooler.
Anyway, I think, after all is said and done, I've sunk in the neighborhood of 1100$-1200$ into this rig. But, then again, I waited for my GFX upgrade to hit rock-bottom, and am a sales sniper, so I've been incredibly judicious with my spending. I do, however, plan on retiring platform soon, and switching over to Core i5/i7 once prices are around 400$ for a full platform (proc/motherboard/DDR3). Or when I pick up a game, and I can't play it fluid at 1650x1080 with good graphics quality.
Oh yeah, I think I've sunk like 450-500 in monitors, too. I've got a 22" Dell that I bought with the PC, and a 24" Gateway (that I regret buying. the diffusion screen for the LCD is terrible, and has a horrendous viewing angle. However, it was the first true Dual Link DVI 24" when I bought it, so I was fine. It still serves well, but for the money I paid, I wanted a better screen.) I Can't imagine spending more than 800$ on a PC for it's first build. Unless I've got a 30" monitor, a 175$ Processor and a 300$ GFX setup is all that's needed to drive most all games between 40-60FPS on 1080p w/good GFX quality. I believe in Buying the bare minimum required and upgrading as needed, though, so what do I know?!