I guess "per calender month".

Yes, sorry; I thought it was a standard abbreviation. Or maybe I've looked at too many rental ads. Perhaps there's a better one?

I guess "per month", "/month" and "monthly" all work well, though I'd like a shorter one. Maybe "pm" or "/m". Or perhaps convert it to years, and use "pa". I'm fussing over this a bit, but I agree with pg that in some cases, conciseness helps readability.

Yes, though Paul Graham's notion of conciseness is not the number of characters, but more like the number of `symbols' or words. So `monthly' should be fine by this measure.