JL kit is excellent and that price is very good for an enclosure, 12" sub and mono amp. By the time you've bought all the components I doubt you'll save much, if anything, by building your own. It's pretty easy to install yourself, but again $85 seems very reasonable if you don't want the aggravation of running the cables.
From what I can tell by doing thirty seconds of research on the product, it looks to be discontinued which might account for the good price. Reviews seem to be largely but not universally positive.