A difficult one, I agree, poteroo.
In an ideal world, user pays - the big end of town who make use of the ILS for training should shoulder the brunt of the cost of maintaining it.
But how to make that work in reality? Charge everyone a landing fee so that us lighties who don't use the ILS end up subsidizing the facilities for airlines and training schools who do use the ILS without landing?
Perhaps user fees (rather than landing fees) is the way to go... or charge the training institutions a lump sum each year, then use that ILS as often as you like...