The chain cranks are expensive when you purchase them separately. And the same costs less on a complete bike. The bike manufacturers deal with the component manufacturers as they buy in quantities which allow them to keep the price low. Additionally, various deals are available for lightly used frames and components. There can be times when building up the structure can be cheaper. Therefore, you can transfer your current frame’s good quality components to the different frames. It is easy to upgrade your bike without spending much money on it.