If you have a group that shares the costs I'd be well and truly.. amazed. In over 15 years of playing rpg's I have NEVER met another GM who's group shares the costs. The GM always ends up shelling out for the new books.
My players buy me stuff all the time but it's more the 3d stuff. But, I know what you mean. For WFRP2 for example, I only had to buy about $300 worth of stuff and there was zero player stuff. That's just how it is.
Don't your players buy you stuff? Maybe you could ask them. I tell my players to buy me stuff they want me to run.
You have you're players BUY YOU stuff? Wow, you got quite the gig working there, no wonder you have no qualms about this seeing that you'll probably get the new Warhammer 3e for free.
Not everyone is as stupid as your players. Some actually like to keep the stuff they buy. I wouldn't buy squat to give to someone else just so someone else could run something. That's called WASTING my money. On the otherhand, if I am going to allow it, and want it, I'll buy it myself as well. People I play with don't waste their money on other people's stuff either.
You have a nice gig going there though. Players desperate enough to have someone GM for them that they'll plunk down their own money in payment to someone else. If you really do it well, maybe you can convince them to buy you a car so that you can drive to wherever game night is, afterall, inflation, bad economy, and all.