I see your point but in this age of internet sales and big box stores that rival the prices of trade specific wholesale houses, both of which make it far to easy for "Joe-Homeowner" or "Joe-Contractor" to check the prices on tradesman such as electricians, plumbers and HVAC techs, I find it easier to be straight up with my clients by showing them my cost and the 20% markup I charge on all the material that I provide. I already do this by including the receipts in the invoices for them to compare but I was asked to include it in the actual invoice portion as well. This particular client is a municipality and with scandals being so rampant and highly publicized, it is forcing the pendulum to swing further and further towards overly documenting everything which for me is no problem as I have nothing to hide and I try to provide my clients with the documentation they require.
So again I can see that for you it may work to simply show your clients the total cost but that doesn't work for me and with the power and flexibility that TimeMaster already has I can't see why it would be a problem to allow the option to show the cost, markup and total on the invoices thus giving your clients more control over their invoices to suit each individual user.
Ok, I will add it to our "wish list". I can make no promises to if it will or will not get done. We have a lot more in-store for TM.
For right now what you can do it do a "Report". For the "Type" change it to Expenses. In the Filter section, choose your Client. Then run the Report from the Start and End Dates. If you have turned on the "CSV" option in the reports (e.g. "HTML and CSV"), it will also include a file that can be imported into a spreadsheet program such as Excel or Numbers. You could send that to your client and if they can easily add a Gross Margin calculation between the Cost and Amount fields.
Now as far as ethics and "good business practice", I don't see how either of those are relevant to my specific question and I think it's important that you keep in mind that what may work for you may not work for everyone else. I long ago came to the conclusion that I will be a small shop electrical contractor with a core group of long term clients and for me, being straight up with them is what maintains our trust level and therefore our ongoing business relationship with eachother. I'm not striving to be Berg, I'm comfortable in my niche of the market and I'd appreciate if you considered guys like me when adding future updates, because as a client of yours that would help me satisfy the request of mine.
I'm just concerned for you! This does not sound like the kind of thing that is common practice at all. Do the supermarkets tell you what they paid for their products? Do the places where you buy your parts tell you what their cost is and their markup? That was my only point, and I do not want to see you get screwed over by someone asking you to tell them your cost. That is very strange! I wish you well in your business and know how hard it can be to be a small company! (that's us too). I'm sorry if I came across the wrong way.

Thank you for taking the time to make suggestions!