I think that should be cheaper, and maybe even better?
Not necessarily less expensive, especially for just building a single. If you build multiples and can get a good multi-channel amp to power the Subs appropriately, then it starts to save money.
Better, you have a good shot.
Especially if you understand what needs to be done to assemble the cabinets properly.
I would argue the true savings will come if you can get your own sheet stock and process it yourself, building multiple Subs with a good quality DSP Amp shared across the Subs.
I seldom recommend that people make their own speakers, but subs are an exception. If you get a kit from, say, Parts Express then you have real value, admittedly with a bit of effort gluing the box together etc.
FWIW, I think what GSG is doing now is pretty stunning. Granted their gear is leaning toward large HT projects in some ways, but getting they are offerering a 1-stop shop for the DIY/AIY person, but also no selling complete builds. For the DIY/AIY-er, if you can find the right box for your needs, their systems are looking to be quite potent.
~//~
Investing in clean and linear Bass is not something one should take lightly. I wish Subwoofers weren't considered the bastard stepchild of the audio world. Bass is not a mystery... It's not a dark art or sorcery. But is is exactly where many breakdown and fail in their system builds.