When I first read the title of the thread, I thought it was intrusive and no ones business, then I started reading it and realised what was actually being asked.
With the exception of a few millionaires and upwards, as have been mentioned above, I would think that the majority of Model Railroaders are just normal everyday people with an interest in the hobby. While it is obvious that money plays apart in the hobby, I don't think you need to be wealthy to be in it or enjoy it. What you have, in term of a layout (to be read as engines/rolling stock/scenery etc) is naturally dependent on how much expendable income you have.
If a vote is needed based on "individual items" for each scale, then I would imagine that a G Scale engine would cost far more than any other scale engine, not withstanding collectibles of course. As such, I would think to own and operate a decent sized G Scale setup would require more finances than to run a similarly sized HO Scale layout. The amount of cash needed JUST for the space for a G Scale would make that scale a more expensive one to become involved in.
I know how much I have spent on my current layout; well actually I don't, but I do know that each purchase usually doesn't get me much change out of $500, and I have a HO Scale layout.
Bottom line to me is this: (theoretically now) I spend $500 on X amount for my HO layout and get X amount. I wonder how much I would get for the same amount of money in O Scale or G Scale or any other scale if it comes to that. It is all relative
Footnote: Obviously, the more expendable cash you have, the more you can afford; however, I really don't think that a hobby should be based on ones financials. It should be based on the persons passion, interest and enjoyment in it. I have had numerous hobbies over the years, RC Aircraft, Modelling (general) and so on. I don't see Model Railroading to be an anymore expensive hobby than many others.