I guess the best bet to is be sincere, and don't be pressured into anything. If you guys aren't able to rent it and it's that important to the bride and groom, they can pick up the tab. Personally, if the rest of the men are wearing black tux's, I can't imagine a black suit will be much different to look at.
When we got married two years ago, we paid for the tux for FOG, as well as the dress for MOG. And we paid for a bridesmaid dress for a family member who couldn't afford it. I don't know what the tradition is in this area, but we knew they couldn't afford it. After having been in several weddings and then being a recent bride myself, I feel like writing a book on wedding etiquette - for the bride, groom, family members, wedding party, and guests. I always get irritated when people use weddings as an excuse for heightened egos...
I hope you post updates, as well as pics of the big day!