Here's are some other ideas about calendars…
You could do one wall calendar and one desk calendar.
The desk calendar could be the sort that is individual cards that fit in a cd jewel case (each would measure approx. 5" x 5.5") so there would be no bindery costs associated. Not to mention, the print cost would be way cheaper than a wall calendar as you could print a lot more per sheet. Jewel cases are super cheap as well. In general the profit on this sort of calendar is much higher than a wall calendar because production, paper, printing and bindery costs are significantly lower...even with the purchase of jewel cases factored in.
For those that haven't seen this type of desk calendar, the jewel case acts as the holder for the calendar...you just flip it open and it become a little stand. I did up a quick mock 3 month pdf version if anyone wants to get an idea of what this sort of thing looks like. PM me with your email address and I'll send it.
Another way to do a calendar would be to do a 4 page quarterly calendar (3 months per page). The size of that could be something like 11 x 17 or small poster size. Again, that way the paper costs aren't huge (and you could use really nice paper) and bindery could be done wire row or some more creative way.
Also, the Illustrator .ai file of calendar month grids can be had for free on Adobe's website...they only have the 2008 version so far, but that one is editable.