I go here http://www.alloscomp.com/bitcoin/calculator
I put in the amount of hashing power I am willing to purchase, EG: 100 GH/s. It tells me I will make $2,094.25 a month. I am aware that the exchange rate changes, difficulty rate rises. But what prevents me from making at least $1500. They say joining a pool is best, but even if I go it alone, what is the thing that will prevent me from making a reasonable amount of money based off of the information in this calculator. I cant be as simple as buying a miner and letting it run all day.