How do you communicate this?
Just tell them. You're the client. You're paying for the service.
Sure if he also getting something from the products I'm buying...
You should have received a detailed quote with prices clearly set out for the service, labour costs and materials separately. I hope you didn't pay the full bill upfront.
What are the possible cost for interior finishing?
How long is a piece of string ........... depends on how much per sqm you pay for materials and what the labour costs are. If you shop around you can often get discounts by negotiating directly with the supplier. For example we bought solid oak flooring for our house from a wood supplier rather than a shop. Much better quality and good price. We used this guy:
With things like bathroom fittings, you can often negotiate a discount if you buy more than one item.