I believe subs are the DIY audio project which offers the greatest benefits.
However, there are some caveats/considerations:
1) the SUB-1200 which is currently on sale for $120 shipped to your home is not made of crappy POS components. I am not at all sure you could buy an equivalent amp, driver, and the mdf, etc. for $120. If you can, you are not going to be "earning" much of a wage for your construction time. But this is an "extreme" product. Of course, most people interested in DIY are going to go for something bigger, louder, and deeper.
2) the more construction involved, the greater benefit from DIY (obviously) ... as well as savings from shipping cost (especially a large/heavy sub that exceeds the FedEx/UPS size/weight limits to require freight delivery).
3) one advantage of most ID subs is they have amps designed specifically to work with the driver and cabinet such that if it is driven beyond safe limits, the sub will prevent itself from damaging itself (often from even making any bad sounds). I think it is a substantial task to determine how to do this if you are building a custom design and do not believe this is common in DIY subs. Of course, one solution is to overbuild it such that it never gets pushed to its limits.
4) DIY offers intangible ($ value not available) benefits such as the pride of having made it with your own hands. The enjoyment of building it, if woodworking is a hobby.
5) The learning experience from making design decisions and (if you use something like mini-DSP) tuning the subs characteristics give you a much better understanding of how sound interacts with you room.
The potential is there for great savings, but one of the confusing things is that DIY includes:
1) Buying a kit
2) Buying a flat pack kit
3) Buying the components from a published design and building the needed box.
4) Working with experienced people to create your custom design based on your specific intentions.
5) Designing and building it yourself
I'm sure I have left a lot out, but my point is there are lots of shades of grey to this topic.