1) It's cheaper to buy than build.
2) It's faster to buy than build.
3) Building usually takes MUCH more time and money than people expect. Winter will be here soon. If you build, you may not have it ready before it gets really cold...or you can spend the winter building as you take your time.
4) Building a bike isn't rocket science. But, it does require some info on the subject.
5) If you build you might also have to buy tools or pay for labor. Those are other expenses.
6) You cannot try-before-you-buy if you build.
The people who really benefit from building from scratch (or near scratch) are those that know what they want and how to get it. Otherwise, it's trial by fire. You'll learn, but it won't be a fast and cheap lesson.