From what I understood, these attacks used DNS amplification. I am no expert on botnets either, but here is the basic idea: they basically send a small request to a DNS server with the source spoofed. The server sends a much larger response to the spoofed source, which in this case is Spamhaus. This happens on those DNS servers that don't check whether the request originated from inside their own network.

So the botnets involved don't have to send 300 Gbps of traffic to Spamhaus. The DNS servers being much more powerful will take care of that. I have no idea about the going rate for a botnet, though.