I don't track it exactly. I have the following-
dedicated miner, 1 5830 (old pentium 4 dell)
dedicated miner, 3X 5830 (dedicated miner built to mine)
guest pc, mines aprox 50% of the time with 1X 5770
main gaming PC, 2X 5830, disabled mining on one card occasionally while gaming
I don't overclock much, get about 270mhash per 5830, so around 1620mhash from them plus some from the 5770... but since my gaming PC doesn't always have both cards mining and the 5770 is offline half the time, I'd guess around 1600mhash overall.
"Each 100 MH/sec would give you 0.05570294 BTC/day. At rate $11.090 per BTC this gets you $0.6177/day " -
http://tvori.info/bitcoin/charts/
Comes out to a little under $300/month for 1600mhash, so seems like the math works out as you would expect.