Yes, developers do charge a huge premium for changes or upgrades. Many builders don’t like to do these alterations or anything the deviates from a standard unit as it requires additional coordination, added time during construction and more chances of mistakes.
It’s best you spend the money on the upgrades that would be very difficult or impossible to change after construction, and do everything else yourself after closing. My mistake was spending too much on builders upgrades thinking it would be easier to get it done and over with but they messed up on most of my upgrades —I eventually had to fix most of it myself as their standards were so poor and they were slow as hell to do anything.
It’s great that this builder has many options to choose from. You don’t see much of that with other builders, it’s often only a choice of one from three preselected finish/colour scheme packages with no substitutions.