The place that is the biggest savings for a complete is wheels. Nobody really looks too hard at the quality of wheels on a bike so they cut corners there. But very few people use wheels like that on a bike they build from components. And they also use cheap chains on a lot of bikes. I went in to my LBS and they had a big pile of chains they had taken off of new bikes because they didn't like the way they shifted. Low-end KMC, IIRC