Perhaps he should be allowed to elaborate before being so quickly dismissed.
Whether or not the claim re: £2000 subwoofers is strictly accurate I couldn't say as no details were provided, (it certainly isn't true of REL's dearer models - they use Volt drivers, which are around £250-£300 each), but it's certainly true that many commercial subwoofers (and not just budget models) are made from cheap parts, which you can often obtain yourself.
One example is the REL Q150, which AFAIK, uses an Audax driver (HT240G0) - available for $53 in the US (£35).
Fair enough, shipping and VAT would add to that if you couldn't source one in the UK, but better drivers are available, even here, for reasonable money.
Our cousins in the US woke up to these facts years ago, and have a thriving DIY subwoofer community.
They may not all be good subwoofers at the end of the day, fair enough, but just because they are home made, doesn't mean they often aren't as good as commercial units - for the same money they are often far superior, and if you pick a particular commercial design, you can often match it's performance for far less money. Many design issues for commercial units are made on the basis of production cost - if you take a typical cabinet, it's not that difficult or expensive to construct something far superior - on a one off run, you really aren't that bothered that it takes two man days to construct - however, this would be economic suicide for a commercial design, even if their superior tooling could cut that time in half.
If you are on the lookout for a bargain £100 unit, then it's probably not worth it - even DIY has it's limits, and it's unlikely you could source the parts for that. However, if you are looking at a £300+ unit, then it's game on.
For someone starting out, they could probably do a lot worse IMO than simply copying the Titanic10 subwoofer, designed by Vance Dickason for PartsExpress. You could order the kit directly of course, but shipping the enclosure half way round the world would add to the cost considerably. Personally I'd do a 12"version though! (not pushing that particular model BTW - it's just one example of many)
Matching something like Eric's HGS18 may not be easy though - access to the type of materials necessary (accelerometer servo drivers for one) would be the major problem. That said, I'm sure there are some advanced subwoofer DIYers who would take on the challenge
. Also don't forget that the HGS18 is over £2000 as well (which is fine if you can afford it/are prepared to pay that much)
One thing though (and this is perhaps one of the big "mistakes" which one might make going DIY) - IMO you'll be extremely unlikely to get this level of performance from a 12" driver and a 200W amp in a little box, all for £300, no matter how good a design you use, and no matter how good the driver/amp are - the laws of physics will most probably see to that.
In other words, while I am saying that you can often get better performance for less money by going DIY, there are limits - you have to be realistic as well.
I accept that DIY may not be for everybody - certain basic skills are req'd, and if you want a furniture grade finish, then that's a fairly advanced skill. All I'm saying is that for someone who is looking to get maximum performance for their money, who also has reasonable basic DIY skills, then building your own subwoofer should not be dismissed so readily IMO - it's really not that hard. It's not as easy as just making a box which happens to fit a space and then just slapping any old big driver in either, but there's plenty of information available to help, often from the manufacturers themselves.
A few links (out of many available) which a budding DIYer may find useful (although you can find these fairly easily anyway with a search engine)
www.wilmslowaudio.co.uk
www.iplacoustics.co.uk
www.partsexpress.com
www.cpc.co.uk
www.diysubwoofers.org
www.bkelec.com
www.adireaudio.com