With any available graphics card today, how far ahead of power costs can you be? My ATI 4870 pulls about 100W, costing roughly twice in power what it mines.
The 4xxx series is the oldest series to have OpenCL support... and the least efficient. That said, it all depends. In Ontario, electricity rates average <0.09$ (we have different rates for different times of the day, as low as 0.06$ at night). If I were paying electricity, I would still break even.