Facilities at conventions are often discounted or essentially free as many cities are all too glad to have thousands of people show up downtown for several weeks during an otherwise slow time of year. Conventions are good for businesses (hotels and restaurants especially) and the tax revenue is good for local government. I have seen where a few places are reassessing whether or not to continue this practice or whether cities can look for alternatives.
With little overhead, conventions have been revenue generators for Watchtower with donations going straight to "mother".
On the other hand, circuit assemblies have the "we have a deficit" announcement that spurred conscientious attenders to feel bad and donate more. With the new per publisher math Watchtower uses, these "deficits" for a one day assembly of one to two thousand people are somewhere around ten thousand dollars or more. And it doesn't matter if the assembly hall is paid for or not.
This FAR exceeds the "deficits" in the 80's and 90's at two day circuit assemblies at rented facilities where the attendance was well above 2000 people. This makes me wonder what is a bigger cash cow for Watchtower, three day conventions where people have to use some of their money into travel and hotels, or one day circuit assemblies at prepaid facilities where Watchtower sets the "deficit" to whatever they want.
It seems conventions used to be for releasing new print publications. People used to feel inclined to put money in the contribution box...and before 1990, they would have to outright pay for the book. Conventions used to come with people having to pay for food tickets and parking as well. All this is in the past.
The recent leaked Watchtower video showed they are looking for any means to cut back and save costs to stay afloat. Whatever changes they are making is in this regard.