I agree that the asking price is a bit steep to reach 28 Hz, although I realize that there's more to subwoofer SQ than extension. And, you have to add the cost of installation to the project. If you have to pay somebody to install it for you, then the cost can be really questionable.
If you can install it yourself, you are quite capable of going completely DIY, save quite a bit of change and potentially get superior performance. But, if your priority is to keep it simple, this looks like a reasonable alternative.