I haven't seen JTR mentioned yet:
http://jtrspeakers.com/captivator-4000ulf.html
Not cheap, but a good value nonetheless.
Factors for choosing subs:
1. How many? Usually takes two to be able to get a reasonably flat response at a single listening position, three is better, especially for multiple listening positions, four starts getting into diminishing returns.
2. How big can the cabinets be? This can affect the type of cabinets, as sealed will always have the least volume, ported bigger, bandpass even bigger, and horns while usually biggest will still be suboptimal(!) size for sub frequencies. Hofmann's Iron Law.
3. How much freedom do you have in placing the subs around the room?
4. How low a frequency do you want to go at what volume?
5. Do you want tactile response- chest thump, the feeling of room pressurization? Which is more important?
6. Powered sub or not? If not, or DIY, what kind of amp power do you have available?
7. Are you willing/able to DIY?
8. Upper extension to cross to main speakers, or other subs?
9. Budget?
Juggle all that...
Multiple smaller/cheaper/weaker subs can equal one bigger/expensive/stronger sub. The reason many of the commercial subs are so expensive is because of tremendous market pressure to keep the boxes small. Smaller boxes often require smaller drivers, which then need more excursion for the same bass. A smaller box means less efficiency and/or higher cutoff frequency. This requires a stronger driver that can handle more power. High excursion, high power, low cutoff frequency subwoofer drivers are the opposite of cheap. What *is* cheap is plywood/MDF and atmosphere. Enough big boxes with cheap drivers will beat the most expensive commercial offering on value- but few people would accept that approach.
DIY isn't for everybody, and whether it saves money in the big picture is debatable. Even if you don't have to invest in tools, education, and experience, you have to figure out if your time is worth it. OTOH, you can plan and build exactly what you want, to look, fit, and perform in a way that no commercial offering can.
I currently run a variety of big DIY subs. If I ever wanted to upgrade, it would probably be to a Double Bass Array. That would fix the number of drivers to eight identical ones, so I'd probably go with 21" drivers as an upgrade. Cabinet volume for the baffle wall wouldn't be a problem, so I might make them all LLT ported. And I'd invest $200 or so in eight cheap 10" or 12" drivers to do a "proof of concept" for the locations before before building the real thing. The only downside might be a lot less tactile response then I have now.