"I believe that you cannot bill a client with both GST and Non-GST components. This is where the complication comes in, you pay for the GST, from your suppliers for goods and services, but when you charge your clients you'll have to include the Cost of Goods inclusive of the GST."
If you are not registered for GST, you cannot bill your client with a GST component. Your invoice to the client will be an invoice, not a tax invoice (a tax invoice is when you charge GST on your goods or service). make sure you describe your bill correctly.
In your example, the cost of that expense is $550 because you cannot claim back from the government the GST component of the cost.
Another business which is registered for GST, then the cost of that expensive is $500 as it will be getting back the $50 Input Tax Credit from the government.
