I've done the math repeatedly... It's not about up front costs, it's about price per Gh/S and how profitable said Gh/S are.
Mobile got it right with the idea of renting Gh/S or trading for them over at cex.io.
At time of writing, CEX.io (the service mentioned above) is 0.1045 BTC/Gh/S. ... This option would not break even in under 30 days (I like tight schedules!), except you have the option to trade Gh/S back for BTC, so your initial investment is "refundable," depending on Gh/S to BTC market rates. You can also continually buy in to further Gh/S with your earnings, compounding your income.
That said, when we're looking at bigger hardware, anything that costs less than ~$6.50/$7.00 USD will break even in 30 days or less at current BTC/USD market rates. However, looking at being "paid back" in USD or other fiat shouldn't be important - the BTC should be.
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.