I agree with Lady Lee and Scully. It doesn't take a whole lot per publisher per month to add up to a tidy sum.
I came from a very "working class" congregation. But it wasn't all janitors and window cleaners. There were a few welders. They lived in a trailer park, and never took vacations or did much to spend money, and they managed to make fairly decent donations every month. My mom and sisters live in an apartment that is about the size of my kitchen and living room put together (and it isn't like I have a big house), and I know that my mom was putting in at least $100 per month years ago when I was in. We had an older couple who died, and they acutally owned a house, because they bought in the 1940's, and they donated the proceeds of their house and what little they had to the Society, and it was talked about for years, because it came to just over $100K. Their daughter was married to a heavy machine worker, who died in an accident at work due to the fault of the company. She received a multi-million dollar settlement and was quite conspicuous about the large amounts of money she donated, and made a big deal in telling everyone about how she donated the money from the proceeds of the sale of her house (which they weren't able to afford until she got her millions) to the Org. when she decided to move to Mexico. And then you had the granparents of my best friend growing up. They lived in the trailer park with about 1/2 the other JW's in the hall. He wore thrift-store suits and they struggled along. His wife had mental problems, and saw a psych -iatrist or -ologist, not sure which, and ended up leaving her husband for the psych. She got DF'd but eventually left the psych, repented, and got back together with her former husband. She ended up getting a windfall from the psych after suing him for improper conduct, and now the entire family lives in a compound and still wears thrift-store suits, and donates a large amount of money each month, I'm guessing out of guilt and to erase her reputation. And then you have the majority of the poor, who always managed to put something in the box every month.
That was in a very lower class congregation. I'm betting most congs. have several odd circumstanses where people have a fair amount of money (at least by JW standards) and one or two people who have semi-decent paying jobs who contribute decent sums, if nothing else for the prestige it gives them. You know even if they don't tell others what they do, the story of how much money they give will get around. And we all know that while they encourage their followers to stay out of evil places like the stock market, that the society doesn't take their own advice in that regard. And as has been said before, they don't have huge outputs in paying people for their labor or do anything in regards to charity work as most religions do.
I totally agree it is a control method, too. How many financially well to do or well-educated people join? (I guess unless you count Brother Prince. How much do you think Brother Prince donates a year? I bet he gives up a decent chunk of change.) You will far easier keep, recruit, and retain the downtrodden than the people with an education, sense of purpose, a decent self-esteem or otherwise fortuitious circumstances. The Org. has been able to keep itself going for upwards of 130 years on the backs of people like this, in a fairly comfortable manner. I think that especially in these financial circumstances, donations might well be down, but I bet the Society has a decent amount of money to live on for awhile. I'm betting that they're not dumb, but maybe a little set in their ways and comfortable with the fact they've managed to have a good existence in the past, so it will be interesting to see what happens in the future with them in this regard.