Both are only taken into account when you sell. Buy 10 shares...

Both are only taken into account when you sell.

Buy 10 shares for $100 brokerage $10 say.
Sell same 10 shares for $120 brokerage $10 say.

Proceeds $1,190 ($1,200 -$10)
Cost Base $1,010 ($1,000 + $10)

Capital Gain = $180 ($1,190 - $1,010) and if held for over a year and you qualify for the 50% discount, your capital gain for tax is $90.

However, if you have other shares bought and sold and have losses, calculating the capital gains is a bit more complex.