If you want to build your own bike, I'd say the cheapest way is to buy a complete tool kit, and then buy the additional tools that you need
It's all depends on your bike as well. Example if your bike BB is pressfit, u're going to need bearing press tool. If it's threaded, you just need a BB wrench.
I personally bought a SuperB tool kit, and then I add cable cutter, masterlink wrench, BB wrench, better quality hex key and a cable cutter. Have build 2 bikes with them, and couldn't be happier..