Here's a profit calculator
http://mining.thegenesisblock.com
The most readily available asics are block erupter blades 10ghs for $200. At $20/ghs the difficulty would have to increase by less than 50% per month to earn a small profit. The problem here is that the difficulty has risen 1260% in the past 90 days.
To profit, you either have to get a lower cost per ghash device, the difficulty has to settle or bitcoins go a lot higher.