Haaa. I work with the 23 year old hot chick too, so if my wife had a problem with it, i guess id have to quit my job.
I am going to answer your question for a completely different angle. Did not read every response on the thread, so apologize if someone already posted this perspective.
Not going to critique how much you pay for your rehearsal space (you like it, you feel it is reasonable for your area), not going to calculate how much you should make as a band (you are adults and can decide if the gigs are worth taking for $ or other qualitative reasons), not going to bash on any band members whether they should be able to afford it (just because I can afford $300 bottle of wine, does not mean I would actually buy it). Instead let's look at it from an opportunity stand point.
If your keyboard player (or any band member) does not want to pay for rehearsal time, then evaluate whether the rest of you benefit more from having that extra band mate vs. the cost of paying to cover their share of the rent. Think of it this way, if Mick Jagger wants to be in your band and does not want to pay $80 per month for rehearsal space, but you get to play Madison Square Garden would you guys think it was worth covering Mick's $80 share? Flip side is if Jick Magger wants to be in your band and does not want to pay, but you play the same gigs / venues with or without Jick, then would it be worth covering Jick's $80 share?
Bottom line it is cost / benefit. If you pay $80 to cover keyboardist's share but make and extra $100 because you can get better gigs then net net you are +$20 having keyboardist in the band. If you pay $80 to cover keyboardist's share and make the same as if keyboardist was not in the band, then you should view keyboardist as a hired cost (like if you were to pay a roadie). Is the extra cost worth the service you receive?
In real life there are some other considerations, such as will the keyboardist be as committed to the project since he/she has no "skin" in the game, is he/she more likely to quit on moments notice since he/she made no monetary investment, etc.
Also if the keyboardist does not share in expenses then potentially he/she does not get to share in profits (when you guys do make money on gigs). Or just for goodwill, the band all gets a couple round of drinks, but after that any profits are split between those who pay for expenses. Kind of like owners of a business, owners have to cover expenses but get to partake of the profits.
Hope the cost / benefit analysis and analogy helps. And if you guys don't want to cover Mick Jagger's share of the rental, please send Mick over to our band. But feel free to keep Jick Magger.