I thought the same thing last week, and started pricing out my diy build. With a 3d printer, laser cutter, and an engineering background, how hard could it be? A simple motor, plastic tube and inlet/outlet.
I was very surprised to find the cost of the acrylic itself. After hours of sourcing and research, I think I could diy something comparable to an avast marine k1 stirrer for $80. And this is with replacing the acrylic body for a cheaper polycarbonate dust collection tube. The acrylic tube (cheapest possible, thin walled extruded) could cost almost $80 itself if you want a 5” reactor. If you wanted a super high quality (cell cast, 1/4” walled) just the tube would cost you $100s.
So yes, it can be made cheaper than it can be bought. But when you factor the time designing, sourcing, and building. Plus factoring the lack of resale-ability with diy vs name brand. Spending $250 on an avast k1 vs $80 on the materials + your time, the off-the-shelf model may be worth it.
Also noting that companies get the materials cheaper, but they also have the added expense of overhead, taxes, and marketing.