I've built a few bikes up from frames. I always spent more than if I had just bought a complete bike - probably 150% of the cost, though I've avoided keeping track of expenses because I was afraid to know just how much extra I spent.

I chose to build bikes rather than buy completes for the following reasons:

1. I enjoy working on bikes. I like having project bikes to work on. On each project I learn something and become a bit more proficient. It gives me a satisfying feeling of accomplishment to ride a bike that I put together myself. It's also reassuring to know that I can probably fix or adjust my bike if something goes wrong on the road.
2. I usually don't have the lump sum required to buy a complete bike, so I buy a frame, then buy components month-by-month when I have a little extra cash.
3. I've always had another bike to ride during the project, so was never in a huge hurry to finish.
4. I like being able to choose the components myself. That's not to say that I don't think you get good components on a complete bike.
5. Sometimes I've been able to find really good deals on components - sometimes from Ebay, sometimes sales at Nashbar or other online retailers. So occasionally I actually end up saving some money.

If you know what models of bikes you like, look for those frames on Ebay. Sometimes people upgrade by buying a new frame and pulling all the components off their old frame. Then they sell the old frame on Ebay.