for other algos then all scales pretty the same so you can chose if you want density for a premium or if you want lot of rig to administer and pay a lil less
If you have more rigs, that means you're paying for more motherboards, CPUs, RAMs, racks and power supplies.
It might be more cost-effective to get more expensive cards.
Nope. Quite the opposite actually if you know what you're doing.
You use a used PC that comes with a cpu, ram, mobo, power supply. Then one medium size power supply for your GPUs.