This usually doesn't work. Oh, it can be done to a certain extent if your willing to wait and scour the internet for eons and you know exactly what you need and want but the official party line is to purchase at a bike store. You will not ever be able to match the current build spec for less than the factory with their massive buying power. If you can settle for slightly less or "outdated" closeout technology you can build a similar bike for close and sometimes less. I'm not familiar with this bikes current price but if you buy a frame from specialized( if they sell it?) that would eat a large portion of your price limit leaving you less to spec components. I can't imagine specialized is unaware of this idea and prices their frames accordingly.
That being said I've built two of my bikes frame up and had a good time. I am decent with my hands and used to work on cars so I thought no big deal, well bikes are not computers and they sure are not cars. I'm not saying it was difficult but I had a few odd ball problems here and there. I was more than happy to pay the LBS to perform/show me the process. I did buy both bikes who's frames were used from the bike store and bought a lot of parts and tools from them so they often gave me free adjustments and advice. Not to mention discounts on the parts. At any rate I'd recommend buying it from the bike store.