I would be intrigued to know if there is any evidence of assembly halls being used as a scam.
What kind of evidence are you looking for? There have been several posters who used to work in accounts explaining the scam. The "expenses" are NOT expenses. The Borg estimates how many people are going to attend (say 2000) and 'estimates expenses' on a per-attendee basis (say, $5 per attendee per day).
That's $20,000 for a 2-day assembly. Actual expenses *might* be 10% of that on the high end. (remember, there are no salaries, no taxes, no mortgage - only water and electricity).
They're vacant a couple weekends a year, so say $20K x 50 weekends and you've got a million dollars a year gross and maybe $900K net.
There is no way this is NOT a scam.
That said, there is no evidence in writing that any known apostate has made available to the public. It is all word of mouth stuff. But sitting there at the assembly and listening to the "accounts report" it should be obvious what they're doing. There is just NO WAY their costs are as high as they claim. And like I said, there have been posters here who confirmed it with their own personal eyewitness accounts.