ill give you a couple examples of why you should limit yourself from true experience from people i've known.
a friend of mine was growing i think 18 or 20 plants in his closet, he was expecting to yield more than enough to pay for rent and smoke. my friend went to work, hurt his back and wrist (merchandise pickup @ sears) and he had to take a drug test for work. he had medical prescription, but his job didnt care too much, but they couldn't let him transfer to any different departments. he decided to quit because he had his plants going. he ended up getting less than a half pound and that was split between 3 people. he lost his job, lost his apartment, and had to stop growing due to lack of money and a place to grow.
another experience, expected 2 ounces per plant figured the math, and it would have paid for everything and electricity perfect. after harvest, he got less than half of what he thought. this means he made half as much money. and means he lost 50% of what he initially thought he would make, had to make a decision to pay the credit card from which he made the initial investment or the electrical bill and rent.
plus, all strains yield different.. some plants are common to get only 10-20 grams per plant, purple kush for example. and other plants such as cheese can get 1-3 ounces. another thing, for first time growers, its rare that someone can actually do everything right, or have the money to afford to fix everything right when it needs fixing. so low balls are just a safe way to not disappoint yourself in the end.