If you have the criminal connections you can buy forged currency in a similar way to how you can buy drugs, the idea being that you can make a return on your investment if you put the work in to shift 'the product'.

Quality is a factor in the price so (for example) a bag of 100 £1 coins might cost £60 but you might not be able to use them in vending machines due to the weight being wrong. Notes again have their price depending on quality and cut.