According to this Gallup poll, vegans and vegetarians compromise roughly 8% of the US population:How big is the vegan market ?
Snapshot: Few Americans Vegetarian or Vegan
Here's a Wikipedia entry that gives percentages for a number of other countries (and cites the above Gallup poll):
Vegetarianism by country - Wikipedia
One might argue that, even if Beyond Meat is too expensive to attract most consumers, they could do okay just by selling to vegans and vegetarians who are willing to pay more for meat-like products, plus consumers who are trying to "cut back" on meat. Is that a large enough market to justify buying the stock? Hmm, I don't know. Food for thought.
The really exciting market opportunity (and environmental boon) would come from selling to anyone who just wants a better burger. But that's apparently expecting too much for now. The Impossible Burger (from Impossible Foods) is also an attempt to achieve that, and may be a more convincing meat substitute than the Beyond Burger, but it still doesn't quite appear to have captured the hearts of meat lovers.