The only ones that had anything close to reasonable prices were the 2012-2013 MacBook Airs. The base one came with 128GB(which was decent at the time) and upgrading to 256 or 512 wasn't a lot more than those drives cost at retail.
Throughout its history, Apple went back and forth between reasonable memory upgrades and ridiculous memory upgrades. They seem to have settled on a general trend recently - if the user can do it, like in a Mac Pro or iMac, charge a ridiculous amount. If it's not user accessible memory, like in a portable, charge a more reasonable (but admittedly still on the expensive side) for memory upgrades.
I bought a tricked out iMac last year and the only thing I didn't upgrade was the memory. Got 32 GB from Other World Computing at the same time for a much more sane price.
