Probably under a grand in the two THTs...Yea my area was so small I just went for first reflections and evenly spaced the other 4 from it. Measuring will probably tell you some things about it though. Are yours 2"? those are nrc of 1.0, the 1" is less so its not 100% absorbtion.
Funny reading through the sub/bass forum guy wants 2 subs for 5000$, because if you spend 5000$ is HAS to be good right? id bet you have maybe 1500 tops into these 2 tuba ht's. Someone I know just recently upgraded from a pair of 4000$ RSL subs, Has the perfect place to hide some DIY subs that would kill most things besides a JTR 4000. I just dont see the whole "I want a pretty sub" thing. When the lights go out and the movie starts all I care about is that I cant localize a crap ton of bass lol, plus...gloss black reflects light. Sorry for the tangent, just needed to get that off my chest.
I know what you're trying to say about the guy wanting to spend $5000 on subs. But, in all fairness you don't know what his space is like. Or, if he has any room treatments. And, typically room treatments don't help bass unless they are VERY thick and floor to ceiling.
DIY isn't for everyone. Some people have WAF to consider; other people just like their stuff to look nice with the lights on.
I had gloss black NHT speakers for years, never had any reflection issues. Especially with the lights out, as you said.
Just something to keep in mind.
Typed on a tiny keyboard, excuse any typos