From what I've read in this forum, the impression I get is around the $500 mark, the performance to cost ratio really takes off, assuming you select the right components and spend some time properly engineering what you want your sub to do. I mean (if I could afford it of course), I don't see why someone would pay $12K MSRP for Gotham g213 when you could make 4 identical well designed $2500 - $3000 subs. Even if you paid half of that and got 80-90% of the perfomance... well, that just makes sense to me
I would also agree with the DIY path not being for everyone. Whether it's the time factor, figuring out what parts to use, lack of woodworking or proper design skills, etc, the commercial market exists and isn't going to go away anytime soon. You just have to be willing to trade off performance for the simplicity of putting your credit card info in.