I made up a simple spreadsheet to see the range of values as difficulty and btc price change. Also wanted to compare for different power consumption levels. Below is two screen snaps.
One for 360W (that's my 2x 5830) and one for 36W (potential 6x FPGA rig).
OpenOffice spreadsheet
here. (updated)
I colour coded it for "good - green" being over $5/day and "give up - red" under 50 cents/day.
Comments welcome, or if you see an error let me know.