Feel like an idiot asking this but..
If I buy something for $10 and sell it for $15...
Let's say for arguments sake income tax is 20%
Not GST registered: $15 sale price - $10 cost price = $5 gross profit = $5 x 0.8 = $4 net profit
GST registered: $15 sale price - $10 cost price = $5 gross profit
15% of $10 cost price = $1.50 gst paid
15% added to sale price = 1.15 x $15 = $17.25 = $2.25 gst collected
$2.25 gst collected - $1.50 gst paid = $0.75 gst owed to IRD
So gross profit =$5 less $0.75 GST expenses = $4.25
$4.25 x 0.8 = $3.40 net profit
Am I missing something?