The issue is not about making a profit in dollars. That mostly depends on the exchange rate. The issue is whether it is better to buy a miner to mine bitcoins or to just buy the bitcoins directly.
Despite what people say, you can get a positive return on investment with a USB block erupter. The block erupter will mine about 0.015 BTC total, so if you buy one for less than 0.015 BTC (about $5), you will make a profit. It is as simple as that, and it applies to all mining equipment.
You forget electricity charges.....
Electricity costs are low enough to ignore, at least for the next several months. A USB block erupter currently mines about $.11 per day and the power cost is about $0.01 per day. Newer mining equipment is even more efficient. When the difficulty goes up by another factor of 10, then the first gen ASIC miners will no longer be profitable (assuming that the exchange rate does not also go up).