there is a threshold cost where you should get more than a box, driver, and amp.
Well, any sub should give you more than that, and DIY is more than just slapping stuff together. Given the design software available, you selecting an amp and designing an enclosure with the characteristics you are looking for. Because there is already a crossover in the receiver, that design issue is taken out of the equation. Where DIY falls short is that most of us are not able to do proper real world evaluation of our creation and then tweek until you get the best possible match.
Where commercial subs offer the most value is in getting a lot of low end performance out of a small package. In the DIY world, the EP800 is considered a relatively small package.
Even there, it is getting easier to do this DIY with a large linear driver and an active crossover/eq box like the Behringer DCX2496.
