Dual CPU mobo for 100USD must be something old slow and power hungry. Stick with your Ryzen unless you actually found a deal which could belong to Great Deals section.
There's plenty of dual CPU motherboards, waaay under $100.
Old - Likely
Slow - Maybe. Quite subjective, depends on the use case.
power hungry - You'll be surprised at the power consumption delta between 10yr
old dual CPU systems, and newer ones. Hint: It's not as much as you think it is.
Edit: As an e.g.
These were recently listed (all sold out now).
Dual CPU = yes.
Old = yes.
Slow = hardly.
power hungry = Dual 2600 v2 series idle ~80w with a few sticks of RAM. Newer systems maybe 60-70w. Not that much of a difference.
Bonus - Two 10g ports and SAS2208 controller included.
Downsides - This is a proprietary sized motherboard (and is huge btw), doing a chassis for this will be tricky.
The point I was trying to make was, a dual CPU motherboard with two CPUs (half decent) and two heatsinks for $27 shipped? I mean...