How much does it cost to monitor [x] websites?

Wormly pricing is based on the concept of sensors - where one sensor can monitor one service (e.g. a website, an SMTP server, Ping a router, etc) every minute.

Different pricing plans offer a different number of included sensors for the monthly price.

It doesn't matter how you group your sensors into different hosts - your pricing is determined only by the number of sensors in use.