Profit Margins on Kitchen Business

Hi there

My son is thinking of starting a kitchen fitting business. I am trying to help him out writing a business plan and need to know what profit margins are possible. He thinks if he offers a complete design and fit service for customers on a £10k job he can make 30% profit - he is planning on fitting howdens kitchens. Is this about right? From my limited knowledge it sounds a bit high but he is saying 30% or even 50% could be possible as he marks up the materials plus his labour.

Thanks