Obviously there are expenses associated with owning a building including ongoing operations and maintenance. Setting aside additional funds for improvements (replacing carpet, etc) also make perfect sense. So, coming up with a rent to charge each circuit is the easiest way to administer it. And since each circuit varies in size, figuring out a total annual cost and dividing by the number of publishers is a simple easy way of allocating costs.
The problem I see is that the WTS gets a cut from each CA. I would bet that's where the increase is coming from. Also, at the AH around here, they've spent tons of money on large flat screen TVs and posted them all around. It's not that big of a building, I find them distracting and don't really need to see Brother Sweats a lot in close up HD!
So they are wasteful with the donated funds and then send a large amount to the WTS who is really turning these events into more of a money maker than they already were.
And the rank and file JW is starting to notice the money grab. I don't think many will do anything about it but it's another straw on the camels back.