Two things to consider:
You would need large amounts of a fairly mild acid to get rid of the green patina. Right now people use lemon and salt in their kitchens, but somehow I don’t see that ramping up to a large scale project. There are all sorts of industrial-mixture acids, so figure that you would be using hundreds of thousands of gallons each month for quite a period of time.
Then there’s the manpower issue – do you build a scaffold, or do you lower the washers from the top? Do you spray? Do you scrub with brushes? What about the folds in her robe? You would almost have to deal with these manually.
And then there is the disposal of the chemical residue from the cleaning.
My guess is that @Espiritus_Corvus ‘s half billion dollars is probably in the right range, if not more.