The price really depends upon what you are willing to put on your bike. There are all kinds of parts, used and new on eBay that are reasonably priced. If you must have new parts, it will cost you almost double the price of a pre-assembled bike right now when all of the sales are going on.
There are many great things about building your own bike though. You will get the things on the bike that you want, not just what happens to be on it when it was sold to you. You will know every little nut, bolt, and washer that is on the bike, no surprises. With the help of your freind, you will know that the bike is put together right.You will gain a wealth of knowledge doing it yourself, and you will have the pride in knowing that you put it together. It is a lot of fun.
The down sides are that it is more expensive and there is no warranty.
I started collecting a lot of Shimano 600 Parts from eBay and ended up with a beautiful group. I then found a chrome moly steel frame that was a little on the small side for me but perfect for my lady freind. I put the whole thing together and it is really nice. Everything except for the waterbottles and cages, tires, saddle, and handlebar tape was used but completely refurbushed by yours truly. The total cost came out to be a little less than $500.00. If this had been a brand new bike, it would have run in the range of $750.00, but I was willing to buy used parts and knew how to refurbish them.