Depending on my perception of the competition, the amount of time I put into it, my work load, and the amount of work involved in handling the order, I use a multiplier of 15% to 20% of the cost (plus tax) as the selling price. Now, for me these multipliers apply to my wire orders for the larger jobs I do.
It is not uncommon for me to have more than $50,000 in wire cost for a marina. If my work load is getting low, I would be very happy with making $5,000 on $50,000 of material costs verses sitting at home or chasing new work at a higher price. I make more money working everyday at modest markups than I do chasing work (which is non paid work until you get a job) at higher margins.
Let's consider overhead. Let's say you are working along and paying (WAG) $15,000/month in overhead. You get a call about a high dollar item that you will have to spend 2 hours on getting the item ordered and delivered. You will have ancillary materials to markup at your normal rates, plus your labor to install all of this at your normal rates. Is your overhead going to go up significantly (or even at all) by buying and selling this item? I'd say no. Your overhead is covered already by all of the "normal" work you do. So this one high-dollar item could be considered overhead free (or close to it).
JMO, it's not a black/white thing. In business, there is a lot of gray.