My spending tends to reflect whether or not I'm actively gaming, and whether I'm DMing during that time.
When 3.x came out, I spent stooopid amounts of monies on books from a wide range of providers - and almost all were hard copy. With several moves in the last few years, I no longer have a gaming group, so my spending has dropped to almost nothing.
I had picked up the core books when 4th Edition came out, just to see what was up - but I was turned off by the system and bought nothing further.
Similarly, I picked up the Pathfinder core book recently - again, to see what they had done with the system. However, Pathfinder I liked so I ended up picking up a couple of the other books as well.
Though, not gaming, I am hard pressed to pick anything else up as I can't justify it to myself - especially with such a volume of 3.x books that I'm not using now.
So, because of those few books over the last year, I think that averages out to the lower bracket of 1-25$ per month.