The mark up debate on parts has and will be a hot topic. Guys that do a bunch of implant work understand what it cost to purchase parts. If they want to include the "Correct" parts than I do not mark up for using their parts. The problem is the liability factor on parts that need to be cast, who pays for them when there is a miscast.
When they ask me why the parts cost so much I tell them two things
My money is tied up in inventory or for a set period of time "terms"
My liability to cover mistakes in lab,
They still don't like it but its part of doing business
In the past I have just wanted to mark up labor higher than my competition and not worry about marking up parts. That no longer works effectively with the amount of inventory I carry now.