As a hypothetical, suppose the camera glass or lens was made with materials or a technique that caused it to be 90% of a phone’s cost to produce. Then replacing this item would cost >90% of the phone’s price, even though it is “small”.
Or you have a huge transport ship that has an issue with a propeller or a small part of the ship on the bottom, but it needs a dry dock to repair, and there are only a couple dry docks in the world that can handle it. Then the price could be a very high percentage of original price due to the extreme supply and demand curves involved.
Repair costs are not a simple a function of the “size” of the damage.
When I say there are people who do the repair for much cheaper I mean including the price they pay for the part, not merely their labor. Your hypothetical is not the case here.
>Of course, the ability for only one entity to source and supply those will tilt the scales towards a higher price
It's not a mere assumption. There do exist people who do many repairs better than Apple for hundreds of dollars less, in the US.