Another commenter pointed out that 2x markup is reasonable, but I want to point out that, without iFixit, 99% of those parts would be useless to 99% of their customers.

I can't count how many gotchas I would have missed, or how much permanent damage I would have caused, if I didn't have their guides to point out hey, these screws are technically slightly different or hey, this cable may as well be made out of gossamer threads with how fragile it is.