No, it's very difficult to assess for a number of reasons:

1) The size of the garrison changed frequently and was never entirely just French soldiers.

2) Local communities were billed for French military expenses over and above those figures for the major fortresses.

3) The requisitions don't appear (to me at least) to have any discernable patterns.

For example, from January through September 1811 Napoleon billed Westphalia 6.1m francs for the cost of French garrisons, the majority of which was presumably for Magdeburg but the documents don't specify that, nor do they even specify French garrisons in Westphalia. Then, however, for reasons I can't identify, Napoleon added a 2.4m franc surcharge specifically for the garrison of Danzig. Why Danzig? It was far from Westphalia, no Westphalian troops were stationed there at the time (although some were en route). I can't explain it, other than: he needed an extra 2.4 million francs from somwhere, and simply decided: "The Westphalians."

These documents are also in Berlin-Dahlem, file numbers: IHA.Rep.,Nr.215.

Regarding Point #2:

I discovered a cache of files in Marburg a couple of years ago, detailing ad-hoc payments to the French military by small communities in Westphalia. The files are enormous. I've examined all of Westphalia's surviving military budgets and I'm pretty sure that none of those charges made it to the Ministry of War and thus never appeared on those budgets. They are quotidian expenses, sometimes very small (14 francs to Captain So-and-So because his salary had been delayed), and sometimes huge (six digits) for broad categories like "Comestibles" or "Sustenance."

A decree of 28 March 1809 made local communities responsible for these costs, meaning: special local "war taxes" that do not appear in either the War or Finance ministries.

At this point I've sort of thrown my hands up and I concede that we will never know the full scale of French extractions of wealth from these allied and occupied states but that it is far, far bigger than the official figures (Napoleon's specific demands for "Contributions.") There were no preprinted forms for this sort of recordkeeping, indicating that it was improvised or unexpected. One department, for example, used forms for the departmental "Office of the Paymaster-General for Various Expenses", having scratched- out the word various and written in war.