Openness and Transparency
The Government's Local Government's Transparency Code contains a number of requirements, one of which is to publicise expenditure over £500. You can read more about the Code here.
The Police and Crime Commissioner's Office also publishes this information here.
The Police and Crime Panel is required to submit returns to the Home Office on a half-yearly basis detailing the expenditure in order to receive the Grant:
Police and Crime Panel Expenditure 2015-16