*disclaimer* I have no hands-on experience with this, nor would I recommend it.
I have seen reports (a lot of TechTubers: TechYesCity among others) who have had success with "cheap*" X79 boards from Aliexpress.
*Relatively speaking, cheap anyway.
$75 or so, IIRC. BUT they're generic (I've even seen reports of boards with weird hybrid-chipsets that simply shouldn't work, but do), will lack BIOS features/updates post "release", have no guarantees, and will take a long time to ship to you (depending on your location).
As for the best "low cost" built, it entirely depends what you consider low cost... along with your intended games/resolution etc.
A Ryzen-based APU *might* cut it, for some people. Less demanding games, low...