The money changers are clustered on the corner of the Parque Isidro Ayora in the Ecuadorean border town of Tulcán. Under the morning drizzle, six of them are tapping furiously into book-sized calculators and peeling off thick wads of dollars.

They are working the steady stream of Colombians climbing out of mini-vans, fresh across the border two miles to the north. Business is brisk and there is little give-and-take on rates.