Hi Jared,
No, an invoice will never show Cost. I have never heard of a client asking you to tell them what your cost is for something. That is unethical and not a good business practice. They should either like your services and prices compared to other contractors or not. I have a long time client who supplies materials for the big builders and they have never asked for the markup percentage or costs. He gives them a great competitive price and they are happy. If I were you, I would tell them that it is none of their business, but that is my opinion. We do have Cost and the ability to show the Gross Margin in the Expense itself, but that is a tool for your use, not for the client. You may want to ask other contractors about this situation, but it does not sound like a good idea to me to give them that information. They could potentially be looking to find out if they should be doing the work themselves, or something strange like that, where your services to them will no longer be necessary.
That being said, I would also say that if you are just getting "reimbursed" for something (without making profit). Like say you were getting someone a coffee and it cost you $2.00, your receipt was $2.00 and you charge them $2.00 then it is no big deal, but of course that would be a $2.00 cost, zero markup.