What you charge for the materials is up to you. If you are happy just to supply the materials you want for no extra cost, then so be it, it hasn't cost you anything and you're still going to earn money from the job.
Personally i would get a price for retail, then the trade price, find the difference, and pass on 50% of this to the customer. Normally, they are happy and you're getting money simply for supplying.
IE Retail price for everything is 150, trade price is 100, difference is 50, you charge customer 125. Hope that makes sense. Good luck, let us know how you get on.:lol: