So, if you do the math correctly, you will see that 1 GH/s will never earn 0.1 BTC, so paying 0.1 BTC for 1 GH/s is a money loser. On the other hand, like you said, 0.03 - 0.04 BTC ($6.50/$7.00) per GH/s will probably earn a decent profit, and I agree that calculating the return in BTC is necessary, and the return in dollars is irrelevant.
When I say "price" for Gh/S, I mean, market price. I can buy them and sell them, and make a profit on THAT as well. Unlike a mining machine, which I can't exactly call the company and return minus a 20% fee (though I COULD pawn it off on some sucker when it's not profitable, but I'm not that kind of person.)
The market price follows the inverse of the difficulty, and the trend is down. Except for random fluctuations and ignorant buyers, you should never make a profit buying and selling (unless you can sell short). Assuming that you don't have the ability to predict random fluctuations, your average gain from buying and selling will be negative. On the other hand, you could just get lucky, but remember that for every lucky person, there is the unlucky person on the other side of the trade -- not everyone can be lucky.