I have a MacBook Pro 13" Retina with 128GB PCIe-based flash storage. Is it possible to upgrade the memory to 256 or more?

Not at this time. I have not seen any third party PCIe storage upgrades. This should change but I don't know when.

The only alternative is to sell your MBP and purchase one with more storage. Good for Apple, bad for your bank account.

Ciao.

You can, but is pretty expensive, see this link:

http://store.mcetech.com/mm/merchant.mvc?Store_Code=mtos&Screen=PROD&Product_Cod e=1TBPCIESSD-L13M14

good luck