A bot net should cost, according to an article I read 2 years ago, around $5-1000 depending on what you need it for and how many. You can get from 500 up to 100k bots. It's rough numbers, but just a hint of how easy and cheap it really is.
Edit: After doing some research, over the span of 2011 to 2017, you may rent a botnet of 1000 for $25 per hour up to 400k bots for $20k/week. Still rough prices.