Bomb Discount halves what you pay for every bomb. Take it if you lean on smoke and voltaic bombs.
Bomb Discount is entry 2 of 13 in the Swindler branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £4,500, 100 Metal, 25 Chemicals. Before Bomb Discount unlocks you need City-Wide Craze.