Absolutely. A cheap, well designed DIY skimmer will definitely beat a high-priced, high end brand name skimmer - at least, in terms of cost. But if your objective is saving money, you're probably not cross-shopping DIY versus the most expensive name brands, bought new. If you're really trying to pinch pennies, you'd probably be comparing a DIY skimmer to a used mid or low end model. And at that point, the argument becomes a lot harder to make.
Also, that shiny new brand name skimmer has an obvious price, it's the sticker on the box. By contrast, it's hard to even talk about cost for DIY. Personally, I don't think it's accurate to just add up material costs for a given end product. Back when the Typhon LED controller project was active, I had probably invested a grand or so into prototypes, mockups, tests, getting PCBs cut, and so on - not to mention new tools I bought, or the hundred or two hours of my time designing, testing, building, and posting abou the project, then working on the software, publishing the design, documenting, and so on. Meanwhile, once the project was published, an average DIY electronics hobbyist could download the instructions, buy the parts for maybe $20 if they were careful, assemble it in half an hour, and have a working LED controller comparable to $150 commercial units. What does it cost to DIY an LED controller? The two grand I spent (plus maybe a couple grand for my development time), or the $20 plus half an hour the hobbyist spent?
Even if you look at skimmers, a scaled down version of that comparison applies. I've built a lot of skimmers, that was a big part of why I got in to DIY'ing for fish tanks years ago. Most only ran for a few months, or even a few minutes or hours - before I tore them apart, replaced them, upgraded them, tweaked them, and so on. Eventually I had one I ran for maybe 2 years. It worked great. It probably had $20 of acrylic in it and a $50 pump. I don't think it would be meaningful to say that I can make a DIY skimmer for $70 though, because if it hadn't been for the prior investment (or copying someone who made the investment) then that would be off by several orders of magnitude.
Then there's the cost of support. If the Kessil on my nano stops working, I have a phone number to call and someone to get mad at. When the DIY LED rig on my old 360g stopped working, well... it was my job to fix it. Sometimes, it can be priceless to have a neck to wring!
I don't mean to monopolize the conversation with my rambling, but I will say that I've done a lot of DIY and put a lot of thought into why I DIY and whether or not my motivations were honest. There were definitely periods where I DIY'd something specifically because I could save money, and it's definitely possible. In some cases, it's even possible to factor in time and effort and still win on cost, once you have the workshop and general DIY skills down, and once you understand the operating concepts behind the thing you're building. But it's a lousy and unfulfilling motivation to DIY. It's kinda like saying that the only reason to have sex is because you want a baby. Sure, some people do legitimately feel that way, and you (might) get a baby once it's all over with, but man, there are definitely other reasons to do that!
. Save the money by putting the sweet equity in, but leave the engineering to the engineers.
