I've read thru this entire thread and I'm still not clear what the topic is, but based solely on the question the header topic asks, I think there is a definitive answer, the Carmody S2000 MT/MTM from;
https://www.diysoundgroup.com/home-audio-speaker-kits/home-audio-size/home-audio-bookshelf.html
It's total finished cost may not be the "Cheapest" (I prefer least expensive, cheap denotes lack of quality), but it excels in three areas that make it a value leader;
1)Performance - On page 4 of Amirm's speaker review, Maiky76 in the process of EQ'ing the S2000 compares it to the Revel M105;
"With EQ on Both speakers:
Very close. Not sure wether the Carmody will play as loud/cleanly though.
The M105 looks smoother and seems to have a better LW..."
High praise indeed.
2)Ease of assembly - The flat-pack is very precisely cut and being diminutive in size, I didn't use clamps or screws, just Titebond II and painter's blue tape.
3)Appearance - Baltic birch, this what sets this kit apart. Assuming anyone one looking for "Cheapest DIY speakers" is not going to have much interest in finishing MDF cabs, finishing the S2000's can be as little as sanding smooth and applying some oil, all w/ no MDF dust!
As someone who is all about budget DIY speaker kits, I have them scattered about 2 houses and, IMO, the OS's and C-notes don't come close to the S2000's. My 8 year old Aviatrix's (Aviatrices ?) are on par, the Carmody's having the better woofers, while the RS28F's of the Aviatrix are still hard to beat as a budget tweeter.