When it comes to mining you have to pay attention to difficulty and think in terms of BTC, not dollars. Starting now, 100 GH/s will earn about 6 BTC over the next year, but the difficulty is rising rapidly and by February that amount will drop to about 1 BTC over the next year. So, figure out how much BTC you are paying for the miner and compare that to 1 BTC you will mine. If the miner costs more than 1 BTC, then you are better off just buying the BTC instead of buying a miner and mining it.
Exactly, difficulty is very important! Everytime I calculated out what I should be making the difficulty would change and I end up with far less per month than expected.