As far as the ridiculous overpricing of train tickets to/from Sydney airport is concerned, If you buy a ticket to the next station down the line at the normal public transport ticket prices, you can get off 'one stop early' at Sydney Domestic and you don't get charged extra. Worked for me.
I haven't worked out a solution for getting into the City from the airport at regular prices, though. You could walk to Mascot, but it's usually raining and that's a miserable walk.