We should generally expend effort on things with non-zero probabilities. Some non-zero probabilities can have huge upsides. Opportunities can easily be found close to zero probabilities. This is because very few people attempt to do things with very low odds of success. The opportunities which are certain to succeed generally attract a crowd. That is why everyone tries to do whatever business their neighbor is doing.
Some things have low probabilities of success but if you succeed, the returns can be quite excessive. However you must be willing to give it a shot with all you have got. Prove to yourself that you have done all it takes to get a non-zero probability.