Go look up the hash rate, or just download gui miner to try things out.
Then plug in your hash numbers into a bitcoin calculator (do a search on your favorite search engine), plug in your power costs from your power bill, and calculate it out.
I've been doing this for a week, and I calculate I will make $67 bucks a year at my current hash rate with my power costs.
That's awesome stuff.
If I bought 2 BFL singles, I could make 10k a year !
I'm seriously thinking about it.