There is a 345 dollar difference between buying the Speedway rear I showed and buying the entry level Summit pieces and assembling it in your case. That doesn't include any tools that you have to buy if you don't have them or have access to them or even the 30 buck how to assemble a nine inch book. Then you have skill level and actual personal time to do the build involved. I know folks who only get 8 hours a week to work on their project cars because of their work and personal schedules. Some of us do have a life outside of the car hobby. We have members who will spend 50 hours and 500 dollars in material to build a part that they could have bought for 200 dollars just to say that they did it themselves. That doesn't count the time and money they spent hunting wrecking yards for the right parts to do it with.
I guess some of us actually like building cars and not just assembling them. Oh and learning new things is good as well.