It’s free to sign up to SIX, with brokerage costs applying to any trades you make including buying and selling. A minimum amount of $500 (set by the ASX) is applied to any trades the first time you invest in a stock. This amount does not include fees so your first deposit will need to be a little more than $500. Once you hold shares in a company or ETF, you can buy more without needing to invest the minimum amount but any new shares you buy will be subject to the ASX minimum.
Each trade you make with SIX will have a flat brokerage fee of $9.90 on trades up to $9,000. Brokerage of 0.11% will apply to trades above $9,000.