I don't see any discount for ordering the "Under the Black Flag" rulebook with the first 15 BG pirates. The first 15 BG pirates sell for $40 as a group and the rulebook sells for $18, while the price for the pirates and the rulesbook is $58. This seems to mirror what was done with the swashbucklers and the Gloire rulebook, the lot of them together was $62, which was no less than the separate prices.
Fortunately I got the Gloire rulebook and the swashbucklers for $55 last weekend.