To update this: it seems that sometime before mid-2013, the cheaper generics finally started percolating out. The earliest mention I've found so far is from May 2013:

How odd, I pay a little under $60.00 a month for for 450XL Watson generic (never any difference for me between Watson and brand).

A Redditor today mentioned it:

If you don't mind, I'm trying to spread the word that COSTCO now has a generic that is only about $170-270 a month depending on dosage (about $800 cheaper than elsewhere). It's made by Watson labs. As far as I can tell only COSTCO sells it right now.