Monsoon, I see what you're saying, but I still say that the cost of materials as viewed by the customer is very different to the true cost of materials used. For a larger job such as yours there may be an advantage and in your current and projected position in development it makes sense to know your costings, but on one-off jobs, such as the OP's, it can lead to misunderstanding.
Some time back I went through an exercise of costing out very carefully what I had actually used over a period of time and what I had charged my customers. I was shocked to find that I had only charged customers for the cable and accessories supplied and that the numerous sundries were effectively coming out of my labour rate. I now have a daily rate to cover sundries, insurance costs, scheme memberships, training, transport, work clothes, advertising, stationery, etc, etc and this cost is factored into the jobs I do. It's part of the cost of keeping me in business before I even start to earn a wage.
So, a quote for adding a couple of sockets might look like this:
Materials - £5.00
Labour - £50
Sundries - £30
Which looks like I am trying to rip you off. Yet £85 for two new sockets is entirely reasonable.