Memory chips, NAND and DRAM, are usually quoted in Gb. Not GB. I WISH flash was at $0.125/GB.
Apple probably pays close to $50 for 128 gigs... Still a huge markup, but it's meant to offset the low (by Apple standards, at least) markups on their base models.
You can look up BOMs for more accurate estimates of product costs and margins... Just keep in mind that they don't include any company expenses (retail, transportation, support, R&D, sales & marketing, management, legal, and so on).