Cheap drinks are always a favorite and people tend to love them simply because they don’t affect their wallet as much and still get them to the appropriate level of relaxation they want to achieve! Remember, though that the RSA course does not condone drinking for the sake of getting drunk, and instead teaches you how to be safe in avoiding this when serving people at a bar or restaurant.
Some cheap drinks that might interest you are:
Passion Pop
Goon (Cheap Wine in a Sack)
Vodka (Cheap Versions)
Max Dry Beer
There are many more cheap drinks that you can buy, but for future reference you should have a look at the liquorland cheapest spirits, linked here! If you would like to serve drinks a little bit more sophisticated than this, get online and complete your RSA course, so that you can serve at a bar around Australia.