Scorpio is a Custom Beast. You are not going to be able to put something together that can Perform like Scorpio
No, but you will be able to upgrade your current PC for a cheaper price than having to buy a completely brand new Xbox model, XBL, and higher average game costs every 3 years. All while having the superior system.
Scorpio is only a much better price if you are completely new to gaming with no new System this gen, but only in the short term. Eventually your XBL, lack of game deals, and having to buy a new Scorpio will catch up to the cost of upgrading that PC you would have gotten instead. PC eventually will close in the cost but with the caveat of being considerably better at gaming.
The brand new PC route WILL still end up a bit more expensive overall at first for this new gamer (not nearly as much as you state by 3 years time) - there is no doubt about this. Better quality costs more in all markets. You're getting all of XB's games on top of several Sony console exclusives and tons of PC exclusives, and multiplats that actually run at 60 frames and better gfx. I'd say a 20-30% more overall cost for the first 3-5 years is worth double the yearly library alone, and possibly visits to the optometrist for having to put up with 30 frames.