I've done three fEarfuls. 15/6/1's coming in at about $600+or- depending on price of the parts. The 12/6 with the alpha 6 mid came in right at $300. (I got a great deal on the 3012LF and bought the Alpha used for $30.) Once you hear one, any doubts will disappear. Once you build one, there's no going back.
All were done from scratch in my garage. I'm a carpenter by trade and I have tools and a space to work. That makes these cabs a fun day in the shop. Having a few bar clamps helps, but you can do this with a minimum of tools. Puzzling out the crossovers, also built from parts, was the hard part for me, and that wasn't a big deal either.
Speaker Hardware has done you the favor of gathering all the stuff in one place and letting you decide how much you want to do for your self. Just having the one stop shopping can save you quite a bit in shipping, as you used to have to source parts from two or three places.
If I had it to do over again, I'd build one, or a pair of 12/6's and call it good. IMHO the 12/6 alpha is the sweet spot and the biggest bang for the buck. I haven't taken anything but the 12/6 out to play for quite a while. It's fairly light, fits in the trunk of a Corolla, and kicks butt for any club gig with PA support and bar gigs without.
I've done other DIY cabs with varied, mostly poor results. The kind of project that starts when somebody gives you a driver and you figure, "I'll build a box for that. How hard can it be?" Greenboy's "shareware" designs have been a huge gift to DIY bassists. The forum at Greenboy's site is full of folks who are happy to help you through a build.
The performance in awesome, simply off the charts for a cab at any price. Find out what a B string really sounds like! There is no need to attempt to re-invent the wheel, these plans will take you through a successful build, and you'll end up with a cab you'll love.
If you want to start at dead simple, a TL 606 can be built very cheaply and works well with a wide array of 15" drivers. I built a pair with 3015's from scraps of plywood I had laying around, each costing under $200. (this was back when 3015's where $150)
Have fun!