This works in both directions. Sometimes you can source a part more cheaply than the estimate, in part because the sourced part is lower quality than what Apple

This works in both directions. Sometimes you can source a part more cheaply than the estimate, in part because the sourced part is lower quality than what Apple uses. Sometimes you can't source a part as cheaply as the estimate, because the estimate is assuming Apple can get the parts at a scale we can't touch. Basically, these estimates seem like wild guesses.