HBO shows don't strictly-speaking depend upon ratings for business success. It's a premium channel costing roughly $180 per year to subscribers. The viewers who actually matter are the ones who would not purchase an HBO subscription without new Westworld episodes. Take that number, multiply it by $180, and then reduce the amount by the production costs.
I believe the estimate is that Westworld season 2 had about 1.5 million U.S. viewers per episode. Let's say 700,000 of those viewers would not purchase an HBO subscription without new Westworld episodes, and let's assume a season 3 cost of $100 million (which was the publicized estimated cost of season 1). $180 multiplied by 700,000 viewers is $126 million. Take away $100 million, and you get $26 million in profit (before accounting for international viewers, DVD/BluRay sales, the long-term effects of keeping a subscriber happy, the public relations buzz for the show, etc.). Add in another $18 million in profit for every 100,000 such viewers above the 700,000.
Alternatively, if only 500,000 or fewer viewers are in the above category, the show probably doesn't make financial sense. Only HBO would have that type of research.