Retail products are substantially over-rated. A fair amount of the cost you pay is marketing and other overhead related articles.
Assembling your own based on knowledgeable advice of those with advanced knowledge is the best solution to obtain maximum value and performance for any given price range.
Or, ignore this advice and buy brand name pre-assembled prodcuts, throwing away money in the process.
-Chris
Not everyone wants to build things. Not everyone is well-advised to try, as there are plenty of people who are not "handy". So for such people, they will not be throwing away money by buying an already made subwoofer, as they are not going to be able to do a good job making something themselves. Also, not everyone has the necessary tools, either, and buying them for one project is going to eat into the cost savings you mention.
Also, there are plenty of places to get bad advice for a subwoofer, so if one follows the wrong plan, the results will not be good. So I think you should give more specific advice -- which actual woofer, amp, and cabinet combination would you recommend for the price of $1k?
As an additional point, the cost of overhead varies considerably. Often, companies that sell factory direct can offer better values than what one normally finds at retail stores. This is not to say that it will not be more than what one would pay if one built it oneself, but there is certainly a good deal of variability in how much money one is "throwing away" for the convenience of not having to figure out what to build and then actually to build it.
And, of course, there is also the possibility of buying something used. This can be the very best value, as sometimes one can get subwoofers below the cost of just the woofers. I did that with a pair of used SVS subwoofers (so if I were to ever destroy one of the woofers, it would cost me more to fix it than I paid for the complete subwoofer in its cabinet). However, what is available used varies from time to time and place to place, and one must be careful to make sure one is getting something that works properly, so this isn't always going to be the best advice to give someone, either. But it is something that one should keep in mind, if one is inclined in that direction.
