Finding the item with the highest markup is going to be pretty hard, but here are some for your consideration:
Any item for a backyard pool - I used to need to replace the gears in my Shasta pool circulator, and they would charge $5.99 for a piece of molded plastic that could not have more that a nickel’s worth of plastic in it.
Appliance repair parts - I just bought a HV diode for my microwave. I figured that it would cost $6. It cost 29, and could not have cost more than .25 to make.
Automobile trim - another category with astronomical prices for simple molded parts.
Years ago, I was eating at a Wendys with a friend, right about closing time. I was drinking an iced tea, and I mentioned to my friend that it just had to be the highest profit-margin item in the place; after all it’s just Tea leaves and hot water. The manager, who was cleaning up nearby, over heard and said “nope.” I asked him what was, and he said “The Frosties.”
He went on to explain how they buy the concentrate for a surprisingly small amount, and dilute it with water, and since they are relatively expense items, they have the highest profit margin of anything Wendys sells.