Hi @JustinB1994 - You can reference other formula columns within formula columns. So could you not just have two additional formula columns (using Total Cost as the name of your formula above):
With GST: {Total Cost} * 1.13 (or whatever your GST rate is)
With Discount: ({Total Cost} - ({Total Cost} * {Discount}))
If you ever need to use formulas but need actual numerical columns for automations, etc check out the General Caster App.
Thanks!
Mark