when I built my two it probably cost more because of the parts I used but the one positive is you end up with the rifle you want. when you buy, depending on what brand and cost, then changing it to suit you might be better off building. it all depends on what you are looking for, a basic rifle or something more. what is it going to be used for etc.
If you buy a complete upper the rest of it is a cake walk. "Its so easy a caveman can do it"