There is already an established economy of botnets. They work like this:
1. There is a hacker group (usually ex-USSR or China) that is actively monitoring 0 day vulnerabilities. They usually target internet connected devices which are from companies that dont give a damn about updating them after they are released. This is the case for most budget routers, "smart" things (lamp, frigde, vacuum, doorbell,..).
2. If they find a nice one (e.g. one that affects millions of shit-tier $40 routers), they set up port scanners and infect as many as they can.
3. Rent the botnet on darknet sites, there is already an established pricing for them, for example $1000 for 1 hour of 1000Gbps DDOS. You just pay them in crypto and tell what to attack.