there's no doubt that if you have a good blueprint to follow, you can definitely build a good skimmer on the cheap. many people have built the randy-style skimmers like those described in the link i posted earlier. imo starting with an established recipe that is known to work well is the best way to build a decent skimmer on the cheap.
otoh, if you decide that you want to start from scratch, the DIY skimmer build can turn out to be quite a bit more expensive. in that scenario, you get to have all of the fun and the expense of doing the R&D work to build a single prototype that will never go into production. in a case like that, when you factor in your time and the cost of mistakes that you make along the way, it might be cheaper to just buy a skimmer.
just some food for thought. for a first time build, i'd consider the randy-style skimmers.