It certainly does cost money to have these assemblies and conventions, even if the building is owned by the circuit or the society and already paid for.
Let's take for an example an assembly hall that was built 20 or 30 years ago and is paid for, and there is a 2-day assembly there 50 weeks out of the year. There is an electric bill, water bill, let's say the parking lot has to be resurfaced every 5 years, seats need recovering every 3 years, the carpeting replaced every 10 years, toilet seats replaced every 5 years. All those costs can be broken down and averaged out, and divided by 50 weeks. Then they can estimate how much $ it costs to operate and maintain the building for each assembly. it does make sense that each congregation or each circuit should pay for their "share" of the expenses. But many JWs can barely afford the gas to get there, let alone the motel rooms.
A more cost-effective idea would be not to have such large meetings. Cover the material on a congregation level, perhaps in special study articles or special Sunday talks. Each congregation has to support itself anyway; why add to that burden?