Why isn't it clear in this day and age weather they are or aren't a financial drain on the country?
Maybe because they'd rather it wasn't clear? After all, clarity can lead to awkward questions which they might prefer not to answer
I think it's more the fact that it is only possible to determine the impact the existence of the Royals have on indirect income from tourism by removing them and waiting a generation or two for memories to fade.
It is absolutely impossible to establish whether the family from the USA who take a two week break in England including a stop-over in London to see the Palace, changing of the guard etc would have come or not if the royals were not a factor - or even the memory of the royals.
The only way of being sure is to remove the Royals, wait about 50 years, and see what the difference has been. Only of course, by then, Visit England and other Destination Management Companies would have invested significant additional money to attract those visitors in other ways.