A recently published IMF paper compares the income statements of 75 central banks and assesses central bank operational cost efficiency across the number of objectives. Cost efficiency measures how efficiently central banks use their (public) resources. The findings suggest that banks with a single objective tend to be, on average, more efficient than those with multiple objectives. The authors suggest that central bank independence, the depth of the financial system, and trade openness affect operational efficiency. Unfortunately, country-level estimates of efficiency are not provided, although annual reports for Russia and South Africa suggest these countries stand out in terms of the relative size of their budgets.
Note that some of the data used in the paper differ from data from central bank annual reports (specifically for South Africa and Canada).