On the one hand, that's about what you'd pay as a share of a three-piece if you guys had to rent an hourly room. On the other hand, you're being asked to pay well beyond the proportional cost of what you're getting, since he uses the room all the time which means you can't just stroll by and jam whenever you want.
The room (or at least, a room---I also live in Chicago and I know there are cheaper rooms out there) is a sunk cost for the drummer. He's paying it whether your band exists or not. So what he's doing by pricing it for you at the point where it is BARELY economical vs. an hourly room is revealing himself to be the kind of guy who will try to extract as much money as he can from his own bandmates.
It is entirely possible that he's having a hard time covering costs and just scraping by, and thus desperately needs every dollar he can get, and sees this as a way to bring some in. But that doesn't change the d-bag nature of it. Nobody should be expected to provide valuable services for free, even to his own band, but at the same time, personally I think it's basic band manners to give your bandmates a better deal than they'd get in the cold harsh commercial world.
For instance, if I provide a PA for a gig, I think I'm entitled to compensation, but I wouldn't charge the same as a commercial rental company; I'd take a fraction (half, two-thirds) of the cost, which lets me feel my valuable contribution has been compensated, but the rest of the band is actually getting a good deal at the same time.
I'd think that $75-100 is a good deal for both of you. More than that is good for him but not so much for the rest of you.
Another approach would be, if he wants you to invest in the room to the tune of 15%, tell him you want 15% of the income the room generates.
