Peppie - You can definitely save money building your own. The materials for my skimmer was ~20$ not including the pump.
I built my skimmer a couple years ago, and Im still pleased with it's function. I googled many designs and watched several videos for different designs before finally settling on a hybrid of many of the best ideas on the net. I recommend do a similar search and building one that suits your needs. Generally, you can build a venturi style skimmer out of large diameter PVC with great success. The good thing about building your own is that the materials are so cheap that you can make a mistake, or start over without losing money. Take your time, plan ahead, and you can save a lot of money on a skimmer if you want.