I'm just looking for an ultra-cheap plinking alternative, or if low-grade ammo is the way to go
If you are already set up and have the brass you can can load much cheaper than factory ammo.
The cheapest bulk 223/5.56 was going for about $3-350/K not to long ago. This same ammo (if you can find it) is now going for $6-700/K.
Components are not cheap either, but better than ammo.
Bullets can be had for about $100/K.
Primers for $35/K
Powder $25/lb, a 1000 rounds will use about 4 lbs or $100/K.
You already have the brass, so if you do not consider your time a cost component, you can still build your own for about $235/K. Don't think you will find any cheap bulk ammo even close to that price.
Not to mention, you handloads will be better quality and most likely much more accurate that the cheap stuff.