Building your own upper really isn't any more than purchasing a complete. Yes you have to invest in some tools but if you are going to be playing with AR's and changing components, your going to end up buying the tools anyway.
The real expense of an upper is if you decide to start getting into precision shooting or better components. If you are looking to save the money, then save yourself the time and just go buy a complete gun. Really, you can get a complete AR these days for cheap.
Be truthful to yourself and what you want to accomplish. Come to terms with what you are willing to spend and build your weapon accordingly. Shooting AR's can be fun if your just plinking but when you start stacking shots on top of each other several hundred yards out from an AR platform, the satisfaction factor really changes.
Here is a couple of my last builds.......
