Depends on the builder.
One builder I worked at had set markups for different elements of the build regardless of prestart/contract/variation. Electrical was the highest markup. Colour selections the lowest. Project builders don't want you to change things, it creates work. It is just one big production line and they want to spit out as many homes as possible, as easily as they can. Sounds harsh, but it is how they make money. The markup on electrical was 60%, tiling 35% and so on. Pretty ridiculous.
Where I work now and the previous high-end builder, we have a standard mark up on all items, regardless of where/when they are priced. If it costs us one hundred, expect to pay one hundred + x.
I won't talk figures since some here know where I work currently.