I don't know any contractors that do not mark up the materials they supply.
It is part of doing business.
Can anyone think of a business that supplies products at their costs?
The amount of mark up varies greatly.
Wire nuts big mark up, expensive items smaller mark up.
Take a walk through the hardware aisle of your local super market, you can find packs of five wire nuts for $4.00, rolls of electric tape so small I would throw them away but yet they get $2.00 for them.
Do you think this is their costs?
