David Schleicher points out that corruption can remain pernicious in larger cities in his paper, Why Neither Exit Nor Voice Substantially Limits Big City Corruption. While lacking data, it does provoke the following line of reasoning: assume that your city has corruption, where are the largest cash flows? How do we currently monitor for corruption for each of those flows? What laws/restrictions exist that constrain the decision-making regarding those flows?
Course, what's good for the goose is also good for the country.