Since there are 3,600 BTC up for grabs every day.
1 (25 BTC Block) every 10 minutes on average = 6 Locks per hour
6 x 24 Hours x 25 BTC = 3,600 BTC.
The best way to determine the amount you will get out of that (roughly)
Your take = (The amount hashing power you are contributing) / (Total hashing power network) * the number of BTC available.
So:
4.5 GH / 500 TH/s * 3,600 = 0.0324 BTC / day
That works out to about 0.972 BTC per month.
That doesn't take into account things like electricity costs or mining pool fees. It does give you a rough estimate or ball part figure.
If you are an optimist you can double it if yo think the network hash rate will only go to 250 TH, or half it if you think the network hash rate will go to 1,000 TH.
Check out my blog for some additional insight.
I am not saying this is wrong but I've been making that on 200Mh/s on slush's pool...