Shop Investor is the top of the income line and pays double the earlier steps. At £20,000 it is the joint most expensive gang upgrade.
Shop Investor is entry 13 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: £20,000, 100 Metal, 100 Silk. Before Shop Investor unlocks you need Tea Magnate.