The trouble with marking up the expensive stuff is that most supply houses around here only have one price in the system for the big stuff, so if my customer wants to find out how much a 6 meter base with 100A main disconnects costs, he hears the same price I pay for it, and then wants to supply it himself. It used to be different - I remember when I could send a customer to the supply house and they would actually charge a markup, then refund me difference between the marked up price and the contractor price. I also remember when they had to know you were a contractor to give a discount. Those days are gone. Now anybody who buys a few lights can actually get a discount on the lower priced stuff. I've even had customers who had better pricing at my supply house than I do after buying just a few thousand dollars worth of lights for their house. That's ridiculous.
If I am purchasing a meter center, I am also purchasing panelboards, breakers, maybe some disconnects or other items all made by same manufacturer. I will be asking supply house for a quote for all of it (and maybe even a few extras for other small jobs) and will end up paying 25-40% in most cases of what I would pay if I went in and purchased same items individually. If I sold for same price as customer had a price on the meter equipment I probably still profit.
Customer needs to keep out of selecting such items anyway. If they purchase this meter center and it is not what is needed - it can easily cost much more than allowing the contractor to get it - there will also be no contractor that will stand behind any defects or other problems with an item they did not sell.
Try walking into an HVAC or Plumbing supply house and ask for a discount price, the price you get may be retail or higher.
Open an sales account there and actually purchase things there from time to time and that will likely change. I have accounts at plumbing, HVAC, lumber centers, and even auto parts stores - most of them give me different price on account than if I were a cash customer, on many items.
Something not addressed here was taxes. In Connecticut we have a state sales tax. Now I mark up my material as much as I can get away with, but my book keeper tells me technically I should be set up at the supply house to not pay sales tax and then charge whatever I want for material and charge the customer sales tax. (Like auto mechanics do). My accountant said don't worry about it it all comes out the same in the end. Just a thought
Sales tax laws are different in every state, so you need to see what applies to where you do business, including other states you may do business in. If you do business in more than one state things can get complicated.
Add: within one state there can be more than one sales tax jurisdiction that needs consideration when you sell items in more than one jurisdiction.