I'm looking to use web search in production, but they haven't mentioned the price. Only thing that's mentioned is $20/month, but how much quota does it include?

Sorry about this. We are working really hard on providing a usage based pricing.

During the preview period we want to start offering a $20 / month plan tailored for individuals - and we are monitoring the usage and making changes as people hit rate limits so we can satisfy most use cases, and be generous.

That's the essence of these services, they never explicitly mention the quota, or secretly lowers it at some point.