If I were a licensed financial advisor, I would say that the $500 (assuming buy/sell in < 1 year) is simply added to your income. So if you are earning $20k p.a. and are paying 22c in dolar for amounts greater than $6k, you would pay .22*500. If you hang onto the shares for >1year, the tax (capital gains tax CGT actually) is halved. So in effect you would pay .11*500.
However, I am simply not allowed to say this (as I am not a licensed advisor, or even an accountant), so I will say nothing.