With giant at least, there are a few frames, aluminum, and usually a few carbon layups across models. Often other things like the wheels and cockpit are cheapened considerably on lower models. It's not easy to beat the cost of a built bike, and building a bike for cost is almost always a constant compromise that comes out very close to the retail of something similar. Unless you are building a unique or custom frame with the purpose of making exactly what you want, it's not worth the work. Especially if you are going to have a shop do the work. Get a 105 equipped bike out of the gate and forget about doing tons of work to beat cost, it's not worth your time. I tell people this all the time, and once I show them the associated costs they usually get it.