I believe this is a factor. Those that disagree fail to realize that Apple rejects sub-standard components. That's a fact.
They also sell a lot of devices with a lot of high end flash. Compare flash read/write speeds in iOS devices to other devices some time.
As to a source for that claim? Can't remember exactly where, would have been 6 months ago, likely Arstechnica from memory.
But yes, if you're adding say, 16 GB of extra flash to say 100 million phones, you've placed a much more massive demand on your flash supplier. From what I've read, Apple's flash suppliers are producing as much of the flash apple use as they can already - if Apple tries sticking more flash in their devices, they will simply have to make less devices in total as there isn't enough of the flash they consume to go around. Thus, they make the devices with more flash relatively expensive - a phone with 64 GB is 3 additional 16 GB phones they can't make... and apple already sell as many iPhones as they can produce...