Coming from somone who has built more than one skimmer, and has done alot of research into them....Your not going to save a hell of a lot of money over buying a good one by building a good one. Of course if you cheap out and build a POS then it will be, however its likely not to work very well and your just wasting your money/time.
I just got finished building a DIY skimmer for my 75g, and while it works great, and to an extent looks pretty darn good too....Im not sure its a hell of alot better than an octopus skimmer I could have bought for a similar amount that I spent...
Look into the Octopus skimmers...they are great, and they are cheap.