If the study of politics is about trying to make sense of what governments do (or think they do) then, surely, it is not only legitimate but necessary to consider the totality of public expenditure. For this is, in fact, what governments do worry about. They do have policies aimed at controlling total public expenditure at particular levels, and these policies in turn affect how much money is spent on individual programmes. To this extent there is, indeed, a common unit of currency: if Department X gets more than Department Y, both the success of the former and the failure of the latter will be measured in millions of pounds.