Rarely (if at all) do any of the "Motorcycle" styles go on sale (unless as another person indicated .. it was a special edition that didn't sell to well). In general, Balenciaga is extremely sensitive about having their items on sale, so if a store/boutique has it on sale, it isn't "advertised" (since they will get in trouble with Balenciaga in that they will no longer be able to carry their line).
In addition, since the Black City is probably one of the most popular styles, it's highly unlikely that you would find one on sale.
As others have suggested, you might want to try a pre-owned bag (obviously in good condition) .. thru the usual outlets (*Bay, Bonz, etc.). Good luck!!