You are right about the russian botnets selling sock5 proxies.
However you are dead wrong about the resources required, creating a sock5 proxy is almost nothing in processing power and you could run 20+ on a standard 512mb box. The issue is finding a legitimate supplier who can sell you more than 8 ip addresses on a single box which you can't do without a valid reason (ICANN's rule I believe).
Most cheaper suppliers (~$1) will be using illegitimate connections, attaching to other slaves or simply reselling the same I.P with different information, or finding a bulk amount of weird located ips such as Romania.