Ah... the 677 from Richmond to Penrith Station via Londenderry... My old nemis...
In the afternoon it comes twice at about 2.30 and 5.10.
If you finish at 6, you are probably better off catching the train, if you are going the Public Transport route.
The price of the bus isn't that bad, well, if you have the bus pass its not. It's $2.20 to Penrith and takes less time then the train, so I would opt for the bus... if it came more often.